Look, all we know is that Riku handed a keyblade to her, and that she put her hand on Master Aqua's Keyblade. She IS a wielder, even if we never saw her summon one herself.
Besides? When was she going to? She was unconcious during most of KH1 - and the rest of the time had RG committee around her so she was protected anyway. There was nothing going on back home. And if she didn't know she could wield one, why would she summon it when Saix had her? She probably doesn't remember Aqua. The first time she had a chance to even try, Riku gave her one (why would he know she could summon one herself?). It disappeared at some point and she as shunted out of the story soon after. Why summon it when she's not being attacked?
That being said (and I don't really want to argue on this point any more) we don't know anything until we get it spelled out. It's too ambiguous at this point.