If you have a lead to getting a letter from a medicine person I would do it. The medicine pool is so large that the letter writers tend to be pretty honest which I appreciate. Plus, in terms of how I think you'll be able to do as a resident in my program, a strong medicine letter can be a boost.
That said, I'm an intensivist and a lot of what I do day to day is much more medicine-based than neurology. Left side/right side, clots vs. bleeds vs. other. But why should this guy be on dobutamine?
And Rearden is right, you want that medicine letter anyway to get the best prelim spot, so you might as well make hay while the sun shines.