Just finished inking some pencils that I got off of Deviantart. They were very "super-heroesque" and it was good practice for me.
Bob Schreck told me one night in a bar in Savannah that you won't ever get better if you don't ink other peoples work. While I don't totally agree with this, I do understand what he means. If you only work on your own pencils or work only on your own stuff, You can get caught up much easier in bad habits etc.
I make my drawings with the intention of the ink being the final work, and not the pencils. If no-one ever sees my pencils I'm fine with that as long as they think my inks are sexy. Having some-one else do the pencils ahead of time takes away alot of the "creator" aspect, but it doesn't make the art any less yours. Inks have a way of fleshing out and finalizing what the penciller has wrought, and making it whole and complete.
Inking over a drawing someone else has spent their time on is very interesting. As I sit there drawing over it with my brush, I criticize some lines, scoff at some blacks, and wince at line weights. But would I do any better? If I gave pencils to someone, what would they think of mine? Inking work is very personal to me, because its like seeing into the other artist's soul, for better or worse.
Ultimately I think that all the inks I have done have bettered the drawing, but does that really give me any room to think negatively of the penciller? I dont know, but I do know that without the work they put into them, I would be stuck drawing on my own...
and maybe even getting stuck in bad habits.