There are studies that indicate an increased risk of cancer with the two meds in combination. Keep in mind though, that the risk is still very very small; it's just slightly higher and just enough to be statistically significant.
Perhaps the more important issue is that new studies seem to indicate that once a person is on remicade/humira, they no longer need the imuran/6mp. In other words, remicade/humira alone is as good as the combination. So why bother taking both when one is good enough?
I have no personal experience in this, and others who do may chime in. But from some research I've seen and a conference I attended, what your GI is recommending does seem to be the trend now.
A report came out this spring that the Remicade/methotrexate combo is actually slightly less effective than Remicade alone for Crohn's. (It works for RA.) So of course my doctor took me off Imuran and put me on methotrexate last week.
No problems so far, but no improvement, either.