I'm not aware of too many dropouts here. I entered UMich in the summer of 1998. There were some folks who entered in 1996 and 1997 that dropped out. 1 had dropped out right after M2 year - he basically was of "questionable character", took the free tuition/stipend from M1 and M2 year and ran. 3 had done anywhere from 1-2 years of grad school and then dropped out - they realized that research just wasn't for them (and their projects were not going well). Since then, I am only aware of two dropouts. Both were from the class that entered in 2000. Again, they did grad school for 1-2 years. They decided that research wasn't really for them.
MSTP dropouts...sometimes this happens. The folks here tend to be more supportive if you gave grad school a try and realized that research was not something one wanted to do. Certainly, the administration will try to convince you to stick it out and finish but ultimately it is the decision of the MSTP fellow whether he/she wants to stay in the program or not.
Overall, the dropout rate here is pretty low. We're really only talking about 5 people out of about 100 total (between 1996 and now).