Way I see it, there's a difference in being able to wield a keyblade, and being a Keyblade Wielder; Wielder being a proper noun in this case. I think proper Keyblade Wielders are the ones who can summon the keyblade, have the powers associated with it, etc. However, I don't think we can rule out the possibility of being able to wield, but not enough so to become an actual Wielder. I think Kairi, being a PoH, has the strength necessary to fight with a keyblade if given one, but can't summon or possess her own. This is probably prone to change in the future.