While not a fan of Dancing with the Stars, it is far from the worst type of thing that could be on the air. Many people bemoan the lack of family-friendly entertainment--well, here it is. No cursing, no sexual activity, no drinking...just people dancing. It's benign, and that's part of the appeal. (Yes, some of the outfits can be a little spicy, but the nature of dance is that it can evoke sensuality--notice I'm not saying sexuality--and if something like DWTS sets you off, you may need to consider ditching the TV entirely.)
Dancing and Idol harken back to the variety shows of old, and provide at least something different than more "boy meets girl" sitcoms or derivative cop dramas.
I don't watch either, but respect that they have large followings for valid reasons.