I certainly don't know the right answer. What I'm reasonably sure of is this: dismissing the idea of May as a bullpen piece out of hand because "starters are always more valuable" is a mistake. As Kab stated above, context is important. I also believe some pitchers perform at a much higher level in short appearances where their strengths can be maximized and the weaknesses minimized. So much of a higher level, that forcing them into starting lowers their value rather than increasing it. I don't know if May is such a pitcher, but I lean in that direction.