I think that had he truly been so weak as people make him out to be based on his personality alone, the Organization never would've sent him to seemingly hound Sora.
We are talking about the brat who took down even Marluxia, and he was no pushover. Sending out a weak member after him would be useless.
Xemnas's sanity and motives might be debatable, but even when AtW called him a fool, he never called him stupid.
Demyx got sent for a reason, and that reason is that he packs the punch, even if reluctant to use it.