I agree... sometimes WWE just comes out of nowhere with some face and heel turns. Some are very successful, some pretty much cost them their jobs. You have some recent heel turns that were very successful, such as Cody Rhodes who really shows a better side of who he is, with his heel turn in mid-2008. I didn't think that Cody could make a believable heel, but he's apparently pretty good. Then you have some heel turns that weren't too good ideas, prime example... Colin Delaney. What the hell was WWE thinking turning Colin Delaney into a heel?
There are also some WWE superstars who continue to witch back and forth between heel and face, some examples are Chris Jericho, Big Show, and Kane. Jericho, In my opinion is a lot better as a face because he shows off a bit more of his athleticism, whereas, when he's a heel, he is more of a mat-based wrestler. Big Show has some face-turn thurns that didn't really need to happen, but he has trouble being successful, unless he continues to switch like this. Since Kane was forced to un-mask, it really hasn't mattered whether he was a heel or a face because either way, he isn't very consistent. I would prefer Kane as a face, but whether he is or not, he still continues to job and job and job... Why?
Now, there are certain people in the WWE whom they would be make huge mistakes by turning them heel now, such as John Cena, C.M. Punk, Rey Mysterio, and Triple H (in his current fued with Randy Orton). They are also some heels who cannot turn face now, most importantly, Randy Orton (in his current fued with HHH). So, I think that WWE needs to keep some WWE superstar's character fresh by those heel-face turns, but it all has to do with timing and how it will affect that particular wrestler in the long run.