It's a good question, and one I have wrestled with too. You can read my concerns about it on this thread from 4 years ago:
http://forums.marrowforums.org/showthread.php?t=447
I still don't know how you can detect any early changes without doing a BMB, but I don't want to submit Ken to it either if there's no indication it's really necessary, and not likely to change his course of treatment. We talked to his doctor some more and he did have another one a few months after I wrote that, just to satisfy all of our curiosities about what was going on. There were no changes, so he hasn't had one since then.