Agree that you don't want to take classes because you think they'll help in med school, but taking courses to help in grad school is a different ballgame. MSTP's are a long road, and the better prepared you are, the quicker and less painless the double degree will be. For example, very few applicants are going to matriculate with just the med school pre-reqs--it'd be really tough to walk into a rotation knowing the bare minimum of science, plus what you gleaned from your research. Rather, most will have taken a number of upper-level biology, chemistry, or related science, and will be better prepared for the rigor of a PhD. I stand by my assertion that MSTP adcoms will be looking for at least a science major, perhaps with a number of other interesting non-science courses (e.g. art history). The argument goes that people should major in the area of their interest. It's easy to picture an adcom member saying: If you're not as interested in science as you are interested in your non-science major, should I really plunk down 500G's to train you for the next 8yrs? With so many other applicants with many more science courses to their name, it'd be easy to pass on the non-science major. The same argument applies for straight-PhD programs (but not for straight-MD), at least in my estimation.