Because that's usually what makes things stale. If a heel is getting cheered...so what, keep them a heel. That'll only make them more popular. Otherwise, you get a case of the 'Orton', or another case of the 'Victoria' where she went completely stale with that crappy face turn. They should start doing with every heel what they've done with Trish. Instead of being a face, she's just become this cocky, arrogant heel, that pretty much tells the crowd to cheer her, as if she's commanding them, etc. There's nothing wrong with a heel getting cheered. Especcially in a case like this. (Now when HHH starts getting cheered over Batista or something, then that's a problem)
And as for why they still bother with the women...because no matter how crap a lot of things turn out, people still go to wWE.com and buy those crazy ass 'OMG CANDICE MICHELLE'S PANTIES!!', and the like. So while they may not be..moving merchandise like crazy, things such as those Diva DVDs, and Magazines, and auction items etc. still sell well.