I think it's okay for her to win unofficial contests (like in Borrowing on Bad Faith!), but actual contests? No. It's not that I don't like Jessie, I really do like her a lot, and I know that she basically is one of Dawn's [not-so-serious] rivals, but she's a Team Rocket member. Two characters going for the same goal at once (and seriously, too) would be a bit much, don't you think?
If Jessie did win an actual contest, it would take Dawn longer to get to the Grand Festival. I think the point in having Jessie enter contests is that so a) Team Rocket can still be included in the episode, because they have to be there...for some reason, b) The writers need another contest rival who isn't seriously trying to enter the Grand Festival, so Dawn can easily win against her and/or if they don't feel like creating another one-shot coordinator, they can add her in the episode/contest if they want to, and c) It's interesting - I mean, who would have thought Jessie would be entering contests?
I'm sorry if this doesn't really make sense. ^^; It's hard to explain my view on this. So I guess to be blunt, I don't want her winning Contests because she isn't really that serious about them and because it interferes with Dawn's storyline.