I don't really care about them myself. It's just that if you're gonna introduce them as something that only TAV can do, then at least incorporate it into gameplay. If BBS had Drives it could've been the equivalent to Final Form and would change your combos to be like LW's DM or something.
I feel you, there's a lot they could have done with the armor to make it feel more naturally integrated and useful in-game. I thought the scene where Terra summons his armor to counter MX's possession was executed really well, and getting to fight in his suit was cool, so I agree that it would have been nice to have that as an established element of the game play.
BbS's biggest problem outside of it's extremely rushed and unrefined gameplay is it's completely weird tone. Why are we playing as stoic, older characters like Aqua and Terra? Where's the fun? There's no wonder or sense of adventure-- everything feels very impersonal and loveless.
I agree with you that the tone of the game is often really dry and stilted, but I think that's more to do with the staggered pacing of each scenario and the sense of excessively brief (and repetitive) world visits which result from that. I feel like TAV are written as forming a real connection to the Disney characters and stories they encounter (some more than others), but because those connections are explored in such a hurried fashion the execution feels flat.
For instance, as a character, Aqua is very human and is always looking to help others as a matter of moral duty: in Dwarf Woodlands, she goes out of her way to help Snow White despite having no reason to believe that it will bring her any closer to finding Terra or Ven because she sees the dwarfs as kind people who have lost a friend, and it causes her to recall her own anxiety of watching over a comatose Ventus. We get (or should get) a sense of her personal history from that. She also has other telling reactions to the plights and circumstances of Disney characters: she's enthusiastic about the power of "true love" which she observes in Prince Philip and wastes no time informing Maleficent that was her downfall, and she senses the good in Stitch and asks to be a part of his "circle of friends" so that their bond will be strengthened. She does explore the Disney worlds with an inclination towards curiosity and wonder: the problem is that outside of some specific examples like those I mentioned, the game relies on a method of telling rather than showing vis-a-vis those constant internal monologues in order to keep the individual character plots succinct-- too succinct, I would argue.
Conceptually, though, I think moving away from a "Sora" archetype and giving us these varied, flawed personalities was a smart move. Terra gets caught up in the villain plots due to his own lack of autonomy while Ven struggles to reconcile his own worldly naivety with a sort of instinctive, deeper sense of purpose he has about things, and Aqua works to establish her own identity and confront the contradictions in her roles as both a master and a friend -- there's a lot to work with there, and I think BBS manages to realize about half of it. But there's a definite sense to me that a lot of the good stuff was left on the cutting room floor in order to fit this oversized package onto the PSP, and that's a shame.
Well with BBS sort of being the turning point and between the two stages of the character vs lore tug-a-war.
I'd say BBS was more character-driven than lore-driven, or at least that's how it was intended. The bulk of the game is spent examining these three characters' different experiences and perspectives within the worlds they visit, and how that informs the decisions they make and the parts they play in the larger scheme of things: Terra's ultimate fate, the literal loss of control over his own body, is obviously an extension of the running theme of rote obedience and submission to powers he views as greater than himself (including his own inner darkness) which colors his entire arc. BBS is actually structured more like KH1 in that it brings the grander mythology into focus towards the latter third of the story but, prior to that, it's primarily concerned with questions of character and establishing how each of their journeys is indicative of their destinations. Though, again, I would definitely agree that its problems with pacing and failure to really demonstrate its themes cause the Disney worlds and the character work done there to feel superficial.