My main problem with this practice is that the credits should be heard and seen normally, as they were intended to. But another issue i have is with the completely unecessary nature of it. It's not like we don't already have TV guides, Electronic Program Guides, the internet (tv.com etc) and of course our own viewing patterns to guide us to what shows are on, and the excuse (which i am guessing is the network's reason for doing this) of keeping us watching, is kind of silly when i'd be far more glued to the tv if they played a good theme tune than if they showed a promo for a reality tv show i've seen fifteen times that day already, and the show i hate anyway.
I've always been a fan of the classic simplistic system of opening credits, show, ending credits, with ads (since they're necessary). In fact i'm not really into previews, flashbacks to previous episodes or 'next time' trailers either, but i think that when you ruin the end credits just to fit in some more network ads it ruins not only the credits but the whole viewing experience. Some of my favorite live action and animated series have catchy themes like doctor who, the simpsons etc and it seems shows are becoming more and more utilitarian in fulfilling their entertainment purpose. We may as well just hook ourselves up to a happiness machine or something.