At the beginning of DDD, Yen Sid made it sound as though the key to defeating Team Xehanort meant that Sora was going to need to go get Aqua if they were going to have any chance of success. Initially, I chalked it up to them needing more keyblades on their side. But, as Lea proved, pretty much anyone with a strong heart can get a keyblade. And, what's more, Yen Sid has the ability to train the in an apparent time-and-space land which makes it easier to to more in less time. So, really, they could get nearly any of the main Disney heroes to go through and earn their stripes, and it would probably be less hassle then having to go bail out Aqua.

 

Not to mention that Aqua doesn't exactly have a great track record of success, so it's not like they're bringing a ton of skills to the table that Sora doesn't already have.

Oh, ok then. Well, still, if it wasnt for the BBS Trio, Riku and Kairi wouldn't have their keyblades, and then what would happen? It'd be all up to Sora alone.

Actually, I'm pretty sure Sora wouldn't have a Keyblade, either. It'd be a totally different group of friends, because the Kingdom Key was originally meant for Riku because of Terra, but since he gave into darkness at Destiny Islands, it chose Sora for its weilder.

They need aqua to find ven gosh,I mean really think about how else are they gonna find ven and terra.also she also the only person who not affected to forget things in castle oblivion.if I remember correctly aqua put ven in there to protect him,and she the only person who knows where he is.how she gonna get terra is goona be hard to predict since he a vessel of darkness so there might be a problem with that

Yes; anyone with a strong heart can wield a keyblade, HOWEVER, there are few who are strong enough---even with a keyblade---to defeat Xehanort. Aqua is among that few. Not to mention...we need Ven back.  3:

Well, think of it this way. If it wasn't for the BBS Trio, none of the current trio of Keyblade wielders(Sora, Riku and Kairi) could use keyblades. (The keyblade wouldn't have chosen Sora after Riku succumbed to darkness if Ven wasn't in his heart, but after Hollow Bastion, Sora has his own keyblade and no longer needs Ven's). So basically, without the BBS Trio allowing them to gain the keyblades Xehanort really would have destroyed the worlds by now to find Kingdom Hearts. They may have caused this(Except Ven, who actually didn't contribute to helping Xehanort indirectly and actually stalled his plans for a time) but if it wasn't for them, we wouldn't have Sora, Riku and Kairi either, and Xehanort probably would have found other people to make him his own vessel and forge the X-blade anyway.

 

Besides, its inhumane of them to just leave Aqua, and to a greater extent, Ven, trapped in their situations forever. Its not right. Besides, the more keyblade wielders they have, the better of a chance they have of defeating Xehanort and having all 7 lights to combat them. For all we know, Ven and Aqua are part of the 7 lights(Im convinced Ven is, since Vanitas is gone, and now he has no darkness in him) and without them, they will be missing 2 people to defeat the XIII darknesses, stacking the odds in Xehanort's favor.

What about Terra

 

1) It'd be sad if she didn't. 2) KH isn't a game that will leave someone with good intentions to rot, regardless of what those intentions wound up accomplishing. 3) Sora isn't a guy who will leave anyone with good intentions to rot, regardless of what those intentions wound up accomplishing. 4) Strength of heart is only a basic requisite for being able to wield a keyblade.  Nomura said there are other factors involved that haven't been disclosed. 5) More manpower=better chances.  She didn't succeed in the long run, but she's a powerful ally to have. 6) TAV have first-hand experience with what Master Xehanort is capable of (not any of his incarnations, the original).  Knowing exactly what you're dealing with gives you an edge.  Mickey can't pull their entire weight in that respect. 7) I assume there are some limitations to that time-space trick making it so that Yen Sid can't stick everyone in there and over-prepare them for the fight.

 This. Plus, I guess, Aqua can explain a few things to the others. Who knows how much Yen Sid and Mickey truley know?

Not to mention that she's a key-piece to the whole 7 Guardians of Light vs. 13 Seekers of Darkness plot?