This is all true but the med school isn't in Cambridge.
Cambridge, Boston, blah blah. It's a bridge that separates the two, not some insurmountable barrier. The academic atmosphere, however, is in Cambridge. Boston is a great city, but it doesn't feel like a university-ish area. I love the contrast between the two.
In any case, I was referring to the fact that, if one ends up going to HMS, you DO end up getting access to ALL the resources in that area. The great hospitals. The research. The communities. The diversity. It's not just the Harvard name that makes the school great; location, location, location isn't just a real estate mantra. Having so many schools in close proximity DOES matter in the sense that there's a real sense of academic cooperation and it DOES benefit medical students. Medical students aren't necessarily restricted to just their medical studies; everyone wants to be able to have some freedom. In an area with so many students, almost anything that student a wishes to do is available.
As for having MIT next door - sure, it's not a huge deal if you're planning on just going into practice (although those great hospitals still matter), but if you have any inclination toward research or other studies, how can it possibly hurt? Having two of the top institutions in the world literally next to each other means that you get the best of both.