I do them sometimes, to keep my sense of anatomy polished. It also helps me to improve on things like perspective...
But I don't do them that often anymore compared to when I was at college. I did tonnes of it back then and now it's not as necessary as it was - I understand anatomy way better now so I've slowed down with the studying.
I do spend a lot of my sketching time experimenting and learning with colour though - that's something I have to focus on since my comics are in colour.
Most of my time is basically spent on actual illustrations and design work nowadays...