Would it not be better to have your best arms as starters, and only after they reach the majors, weed out certain ones to be relievers?
I think you underestimate the difference between starting and relieving. With starting you pretty have all day to get ready both physically and mentally. With relieving you only have a short period of time to do so. IMO with a young arm you want to get it used to a routine and have it stick with the routine. I don't know how long it would take for a player to get used to switching from starting to relieving or vice versa. Also wouldn't you want the players you had pegged for a reliever to get used to pitching in that situation?