Two professors I know, including my PhD advisor, both dropped out of the MD part of MSTP programs to pursue only the PhD. The first was at UCSF, did the preclinical years, but then got really into his research and didn't want to drop it to do the clinical training. He's happy and successful in his career now.
My advisor started out in Stanford's biophysics PhD program, decided he really wanted to do the MSTP and somehow convinced them to let him in, then dropped out after another year because he thought all of medicine was like the MICU (desperate, futile attempts to prolong the suffering of old people who were going to die anyway, was how he put it). Although perhaps not a well-informed decision, I think that too was the right one for him. He's now an HHMI investigator.