I feel like the only way a person could see the drop system is unfair is if they ignore all the benefits and focus on the only the few perceived negatives.
Drop Bonuses are hugely powerful at lv3 trust me I forget I'm playing critical mode when I max out my bonuses on lv3 until I lose the gauge and then man do those sudden drops HURT really hammers in how much it sucks. Oh and don't even get me started on how easy it is to mass open dream eater grids with the poorly named "affinity boost" which boosts everything your dream eater receives not just affinity. Food + Affinity boost = 1,000 or more LP in minutes and since the game's main item it throws at you is food there is really no reason to not use these. With boost this big there does need to be some sort of limit, the limit of course being the time limit on the drop gauge. It's no different than say eating a meal in FFXV you get a boost and it only lasts until the gauge runs out the difference here is they are nice enough to let you keep the boosts indefinitely so long as you restore your gauge. Which I can't stress how utterly effortless that is to do.
It's a hugely beneficial element and it adds an interesting mechanic where in Riku and Sora can actually actively help one another in a gameplay sense while not being physically together in the same world. You know we always have these stories of friendship and how the characters are so close/always have one another's back. But it's not terribly often we get to experience that in a gameplay sense as anyone but Sora with his DG/Disney party member system. Sometimes ever so rarely we have cases where Sora and Riku can fight teamed up, TAV can fight teamed up, and RAX can fight teamed up but these are far and few. TAV did have the wayfinders but the D-link system was kind of poorly implemented and doesn't quite capture the same feel as it felt more like you're impersonating your friends rather than them fighting alongside you.
The drop system is what keeps DDD from falling into the same trap. Every enemy Riku kills gets his drop points to spend strengthening Sora. If he spends the points on abilities and items Sora gets access to them to, and if he converts it into money Sora gets access to it to. They don't share drop bonuses so when they buy bonuses they are are sacrificing the ability to get stuff for themselves in order to support their friend. It's a gameplay system that really lets you feel the bond between the two, they are apart and yet protecting and giving one another strenght every step of the way. Likewise Sora and Riku share dream eaters so they can work together to train a dream eater faster or divide and conquer by training different dreams so they get access to a larger variety of things. They share inventory, they share commands, they share items, they share everything and the drop system is the intrinsic part of all this. It really feels just awesome to make progress as Riku, drop to Sora, make more progress as Sora then share the benefits of that progress with Riku (since you can edit the other's deck and share the same dream eaters at any time), so when you drop back to Riku and he wakes back up he wakes up stronger than he was before and can have an even easier time continuing his progress through the worlds than he did when he went to sleep. The two sharing their experiences at all time is just great.
Likewise the drop system allows the players an unprecedented amount of control over how they progress the story. You can for the most part play Sora and Riku's stories separately like in CoM or BBS doing all of Sora's stuff first then all of Riku's, there are only a few blocks where you can't progress and less both have made it far enough. But you could also take it by world so if you want to see a whole world's story you do it as Sora then immediately drop and do it as Riku. Or do the same world simultaneously dropping back and forth between the two to see that one world's story playing out in alternate parts. You could also just go with the flow and let yourself drop automatically and thus view the story as a flip of a coin. The drop system allows us basically total control on how we view the story and we don't need a god damnable flow chart and 5 different save slots to follow the plot chronologically unlike freaking BBS.
Then there are all the other miscellaneous gameplay and story additions it brings. With the drop system we have then the forecast system and dropping lets you adjust the passive benefits you will get during that world.
-One that makes you get double money from everything
-One that makes yours and enemy magic attacks stronger while speeding up your gauge
-One that doubles the HP orbs dropped by enemies
-One that increases the amount of prizes dropped by enemies while also speeding up your gauge
-One that has no passive benefits for anyone
-One that makes shops sell rare items and gives your a 20% discount
-One that makes enemies stronger, doubles your gauge speed, but effectively reverses the chances of crafting rare dream eaters so instead of a 10% success for a rare with an 80% chance of getting the regular it becomes an 80% chance of getting the rare and a 10% chance of getting the regular.
So you can drop freely using the drop button until the world will have the passive effect you most want on it. Meanwhile on the gameplay enemy/lore side it makes the dream eaters even more unique and have more of an identity than just being "heartless substitute v2.0" which was something the Unversed lacked. Dream Eaters specifically nightmares are all about putting things into never ending dreams so it makes sense that quite a few of them including all the bosses (minus the ones that happen after the gauge is temporarily taken away) have sleep attacks that instead of directly putting you to sleep hasten you to sleep by speeding up your gauge. And when the gauge empties you'll fall asleep and next you wake those same dream eaters will be back again to try and put you to sleep again, all actively trying to prevent you from freeing the worlds from their slumber. It's a unique little bit of identity and and it adds an extra way to effectively "lose" where they can exhaust you physically through your HP killing you or exhaust your stamina through the drop gauge. It's an element uniquely theirs, their ability to make you lose through an eternal stalemate via sleep.
This has another effect which is keeping the gameplay fast paced, as someone who has personally tested this nearly every boss in the game (and ALL enemies in the game) can be beaten in under 2 minutes and none should take no more than 5 minutes discounting cutscene time. It encourages the player to fight at a very fast paced instead of a slower and more methodical one. The bosses are based around this, the bosses are all designed to keep you moving and attacking. The idea is to keep the battle flow going and with how mobile and powerful Sora and Riku are it honestly makes every fight almost feel like you're playing KH2 Critical mode. A mode which created a similar fast paced battle flow by greatly upping Sora's damage while lowering his max HP and MP so it quickly becomes a fast paced battle of kill or be killed. I love DDD's battle flow you feel like such a freaking badass rushing around while the gauge ticks down and trashing these giant foes in minutes. Likewise it's a good way to punish players who are taking too long in their fights. But drop me-nots, drop me-nevers, break time, and wake up block all exist and let you engage in a very different battle where slower more methodical players fight flow with flow.
The drop gauge is in conclusion not only something that improves the story both through giving a tangible way to see character bonds and the individual identity of the dream eaters from other enemy species. It adds significantly more ways for the player to control their story progression and how they play the game. It adds a fun and easily manageable gauge truly no different than any of our other gauge based systems with immense amounts of benefits and pretty much is the perfect balance. Finally it's a core part of DDD's identity and should definitely never ever be removed from DDD, in fact for all future games dealing with multiple playable characters I really do believe the drop system should be utilized. Maybe modified to fit that game's thematics and that might mean increasing the time or coming up with some other kind of method of gauge management.
