[Baalz]

Yeah, outside of mists of deception combo it seems
1) Like a valid tactic
2) Impossible to even tell for sure if the mage retreated on purpose
3) Considering it an exploit would make things like the horror harmonica completely unusable. To a lesser degree lots of stuff is much harder to use if you have to commit to *never* withdrawing the caster.
4) Not obvious - you can't expect everyone in a MP game to have read this thread. Most everyone does this now and it doesn't seem exploitive, people are going to continue to do it because they don't consider it an exploit.

So, IMHO, good thing to fix as a bug but outside of mists of deception combo a valid tactic.