I just went throught the exact same thing with our GI. She said, that she feels the Pentasa isn't an effective drug, and to continue with Imuran only. My personal contention was why mess with this combination if it's apparently working. I guess it's a matter of opinion. She did relent abit, and suggested we cut the Pentasa in half for a few months before eliminating it.
Also, Imuan does take quite some time to reach it's full effectiveness. Up to 6 months.