Some folks think that she was shoehorned in. The creators of the show were creeped out that a lot of fans were shipping Ben and Gwen (who are COUSINS, for Christ's sake) together, so they brought Julie in.
Personally, I always liked Julie. Incest pairings, like BenXGwen and NejiXHinata, have always disgusted me.
Sadly, it seems like they were never serious about making her a major character, what with the few times she appeared in Alien Force and Ultimate Alien, so I wasn't surprised (disappointed, but not surprised) that they almost completely wrote her out of Omniverse.