Hmm. I just rejoined the forums; I haven't been around since the fancraze that followed KH1. I haven't had time to post in the New Members Area, to add an avatar or a signature, or to post up any of my personal theories, but....I felt that this topic needed a little help. So...*KA-POOF*....here I am.
Ok. As far as I can tell, none of your conjecture about Roxas's keyblades being "shells" or "fakes" has answered the original question asked. Just to refresh all of your memories:
Roxas was a nobody, who just like the rest of the nobodies in the org, who wanted to be real, or to exist. Saix said that each heartless slain by the keyblade adds to kingdom hearts (as long as they collect the hearts). Once kingdom hearts is full of hearts, they can use the power to make themselves "real" (like full beings). Roxas had the power of the keyblade right? Why didn't the org just summon loads of heartless, ask Roxas to kill them, collect the hearts, and repeat the process again.
The answer to this isn't all that complicated. All you have to do is refer back to the last game, and be attentive to the timeline of KH2.
Point 1: Are Roxas's Keyblades Fake?
~ Really, it all comes down to personal opinion. In MY opinon, the answer is no, simply because I see no reason for a fake set of keyblades to be created for Roxas. Does it take a heart to wield the keyblade? Yes, but we know from the Secret Ansem Reports that Roxas and Namine are incredibly special Nobodies, capable of things that Nobodies simply shouldn't be able to do. In the same way that Namine can modify the memories in people's hearts without having a heart herself, Roxas can wield the keyblades without having a true heart.
Point 2: WHY is Roxas fighting Heartless in Deep-Dive?
~ When you think about it, Roxas fighting Heartless in DD most likely has little to do with adding captive hearts to Kingdom Hearts. As we learn in KH2, the Nobodies and Heartless have become enemies due to their allegiences to Xemnas and Maleficent. This most likely occurred while Sora was sleeping. As such, an obvious goal of the Heartless would be to take over Organization XIII's base of operations. What Roxas is doing in Deep Dive isn't part of the Org.'s master plan; he is simply clearing out invading Heartless.
Point 3: Why can't Roxas massacre the Heartless whenever he wants?
The main reason for this is because Organization XIII could not summon Heartless back when Roxas was a member. But that will be explained in the next point.
It was mentioned that Sora was able to fill up Kingdom Hearts in a short amount of time. How was this possible? You see, Sora was able to fill power Kingdom Hearts because the Heartless hunt him relentlessly. As mentioned in the previous game, the Heartless have great fear of the keyblade and seek to destroy its users. With so many Heartless coming at him constantly, it was easy to kill a lot of them.
Ok, soooo.....Roxas has the keyblade, right? Why don't Heartless track him down? Well, the reason behind this can be traced to a cutscene from the beginning of KH1. Sora is sitting in a hotel room with Leon and Yuffie, and notices his keyblade sitting across the room. Yuffie explained to Sora that they had to get the keyblade away from him to shake off the Heartless. Whenever the keyblade is in contact with Sora, the Heartless can sense Sora's heart.
And there you have it. The Heartless DON'T hunt down Roxas because he DOESN'T have a heart.
Final Point: Why can't Organization XIII summon Heartless for Roxas to kill?
This one is probably the easiest to explain. Xemnas DID NOT have control over the Heartless back when Roxas was still a member of the Organization.
To quote Xemnas himself, "...the Heartless ally with whoever is strongest."
Organization XIII did not gain control over the Heartless until the cutscene after the 1,000 Heartless Battle, when they proved that they were stronger than Maleficent. Afterwards, the Heartless realized that their leader was weak, and allied with the Org. By the end of the game, Maleficent and Pete had so little control over the Heartless that they had to resort to fighting them in The World That Never Was.
.........
Whew....I hope that explains things well enough. Anyone have any questions, comments, or disagreements?