Cole had enough money that he wouldn't have to worry about it even if he flamed out. Not only were his parents rich, but the Yankees offered him a huge deal. He actually risked losing a lot of money by going to college. Though for many people it would make sense to go to college, especially with the new CBA that limits draft bonuses.
As for the initial question, yes they suck, because most of the best relievers in MLB were at least good enough to start in college or the low minors. College relievers are a lot less likely to be potential MLB prospects, I would think.