I recall my class had a student who had withdrawn and re-enrolled twice during the pre-clinical years. We had none that know of who willingly quit, though I'm sure on paper most (if not all) of those who were "asked" to leave will be recorded as voluntary withdrawls. I'm not sure what the impetus is to get students to "quit" rather than just expel them, but residencies do the exact same thing so it must be something to do with the lawyers.
I was miserable in my pre-clinical years and desperately wanted to quit. I couldn't, since I was on military scholarship, but if I had had any other options I would have.
My medical school was very good at supporting the weak students, and very good at the whole touchy-feely hugging thing. They were not good at dealing with someone like me. I did not understand why my aggressiveness was discouraged, or the desire to do more than what was offered was considered some kind of sin. Everything was focused on public health and primary care, and any suggestion that you weren't considering these fields was something of a heresy to the regular teaching staff. It was all very . . . and forgive me for saying this, but I can't think of any other way to describe it . . . feminine.
After enduring nearly two years, I was emotionally done. I explored options on how to withdraw without ending up as an unrated Seaman chipping paint on a destroyer, but there were none. Fortunately I stuck it out, because I blasted Step 1, got my specialty of choice, and am moderately satisfied with my current surroundings. Still, if I could go back and do it over again, I would never have started in the first place.
I guess what I'm trying to say is that I have a world of respect for people who quit this field, especially when they are in medical school, with all the financial and emotional pressure trying to force you to stick it out. Anyone who decides to bail in their pre-clinical years has my undying admiration.