I think the fact that Seifer had always dreamed of being a soceress' knight and that Ultimecia needed someone strong as a general lackey, Seifer just proved to be easily manipulatable and relatively strong. Obviously as the storyline goes, Squall and co will beat Seifer, but there's nothing to me to suggest that anybody other than the playable characters could beat Seifer. After all, he manages to beat Odin. So I think it was a combination of Seifer simply being second best to Squall's gang which makes him appear weak. From Ultimecia's point of view, you've got a strong servant and a willing one at that.