Why isn't Lee valuable as a role player off the bench?
He absolutely would (will?) be. But we can manage without him.
But Lee is not going to ever be a first round pick and this draft sucks. I did not think it was stupid for him to try it and had he performed better, it might have been the time to go. Coming back won't have any effect on his pro prospects, frankly.
On the other hand, Briscoe is a legit NBA player once he gets his shooting in order. That is between his ears. It makes a world of difference to UK if he comes back and it makes financial sense for Briscoe, too.