Myles is a wimp. He pretty much will not go anywhere beyond Arnika and Trynton. When I play again I will definitely not recruit him and keep the Exp for myself.
As for an explanation, any RPC will give a little speech about why the do not want to go into a certain area.
Vesselle
I knew Myles was a thief but I did not know he was that much of a thief. I think you should go right back to Arnika and show him who's boss.