Centrifuges in space are not a solved problem. AFAIK there has been exactly 1 experiment in space with artificial g, at very low levels of g. You want to be able to maneuver and orient your ship even while it is spinning, and you want to study the failure conditions (most plans call for two tethered modules held by cable -- you want to experiment with what happens if that cable under tension breaks).

Also, we just don't know what 3 years of Martian gravity does to a human. It would be good to see just how much better 3/8 g is against 0 g.