Buizel should evolve, but only for defense. Just because Buizel beat it's evolution doesn't mean squat. I mean, just because it's strong enough doesn't mean it can't or shouldn't be stronger.
I mean, if it's strong enough to beat it's evolution as a Buizel, how powerful could it be as a Floatzel?
And after it's defeat against Ursaring, it needs defense. Disregarding the fact that Ursaring is a total beast at just about everything, Buizel should be the strongest on Ash's team (minus Pikachu), judging by the number of wins it has.
But do I think the writers will evolve him? No. If Buizel evolves, Ash will have a semi-large Grotle, a large Staraptor, a large Floatzel, a large Gliscor, a large Infernape/semi-sized Monferno (depending on if it continues to evolve), and a bite-sized Pikachu. Ash always uses plush-toy Pokemon, and even if he's breaking the mold of "small-but-fast" teams, he still won't get a full team of complete powerhouses.
Unless this is his last journey ohnoes!