Spades

Because his heart and Xehanort's are interlocked. If you remember when Aqua and Terra-Xehanort fight he can only lock away Terra's heart by locking both of theirs and in blank points he claims that his heart will eventually engulf Terra's completely. And if you remember the original Xehanort is only able to return because Xemnas and Ansem SoD were defeated. Allowing his heart and Terra's to return to his body.

That's fine. I'm just speculating after all. He can still end up being one of the seven lights but i'm kind of hoping he doesn't. If nothing for the interesting conflict it will bring for Aqua and Ven if they have to choose between protecting the light and Terra.

Where does it say part of Terra's heart is in Riku? I haven't really watched all of the videos so i'm just curious. And I don't think part of his heart resides in the LS. Wasn't it just animated by Terra's rage at Xehanort? "Your body submits, your heart succumbs, so why does your mind resist!?" And as Xehanort is kind of demonstrating dividing your heart doesn't really seem to weaken you all that much if at all.

I know he was supposed to be one of the lights, but that's the rub. Xehanort himself said that Mickey would have to find three more because two of the lights (Terra and Sora) were going to be under his control and be on team dark instead. Terra is still under Xehanorts control and is one of the thirteen seekers of darkness instead. Lea is the obvious one to take his place.

Okay so we all know about the seven lights and the thirteen seekers of darkness but as Xehanort all so politely pointed out for us he had already corrupted one of the seven (Terra) and was on his way to double up on his corruption. This pretty obviously removes him from being one of the seven with his place going to Lea. But I really doubt Nomura is going to have him do absolutely nothing in the final battle, honestly I think Terra might end up being the most important player in instrumenting Xehanort's downfall and here's why. Xehanort's biggest advantage (being able to split his heart and fill other vessels with it) might end up being his greatest disadvantage. Terra's and Xehanort's hearts are interlinked, while he was always in control he was never able to completely banish Terra as blank points showed us. Instead Terra is biding his time until he can win his body back. When he divided his heart to fill the other vessels it goes to reason that he divided Terra's heart as well. If Terra can win control back of his heart even just for a brief moment he might be able to gain control of not just his body, but every vessel that Xehanort filled his essence with. Xehanort told Mickey to meet him in that place (The Keyblade Graveyard) when the lights were assembled. Who else do we know of that resides in that place? None other than The Lingering Sentiment who will probably be up for smacking around some norts, and as a result of it merely being Terra's armor it would not count as one of the seven lights, helping to even the odds a little. I can see Terra waging a battle on two fronts when we get to the climax. His Lingering Sentiment helping Aqua, Ven and the others on the physical plain, while he and Eraqus battle Xehanort in a dive to the heart, trying to wrest control of the vessels from him. Master and pupils defeating xehanort together. So what do you think?