My wife has been on 6MP for 6+ years. She had both our kids on that drug, so for us, it's a wonderful drug.
Any type of chemo can cause some form of leukemia, so they have to label that as a side effect. I wouldn't worry too much about it.
Like I told my wife, would you rather deal with crohn's now, or take the slight chance of something else down the road?
They have you on 6MP to keep your body in check so it doesn't attack the Remicade. Donors use 6MP or some form of it as well so the organs are not attacked. Usually, a very mild dose and definitely nothing to worry about.