In general this was a great post, but I'm not sure I agree with this statement. I spent a few days in high school shadowing a friend of the family doing his residency, and then just talked to some other MDs and that was enough for me to decide on medicine...thus ended my clinical exposure. When I applied I had high numbers, lots of research, and no real volunteering/clinical exposure. The doc interviewing me at my future med school asked me if i ever shadowed because "some people on adcom make a 'thing' about it" I told him I did a bit in HS and he said that was fine, he just wanted something to tell the rest of the committee...then he told me clinical exposure is overrated, the only thing it really tells you is whether or not you'll faint at the sight of blood.
Now, I'm not saying that volunteering can't be a very rewarding experience...I tried to volunteer over summers in college but the local hospital told me they "weren't taking volunteers" at the time (don't want my free help?!) And yes my school is somewhat research oriented, so that might have contributed to the adcom's point of view. I'm just saying, maybe we overestimate the importance of "sustained clinical exposure." It is certainly not the only way to answer the question of "why medicine" my 2 cent...