Another factor is that some of them might turn into closers if the team is desperate. Lidge is an obvious example, but Ryan, FRod, KRod and Gordon are all guys who, whether it injury or inadequacy, could get you some saves down the road.
Also, if you have a dominant MR, their weekly stats can compare to a starter. Ryan can pitch 4+ innings a week, usually 1-2 Ks per inning, and maybe an ER. That's 4 innings, 6 Ks, 1 Er. With maybe a win involved, that's not a bad line to have in there.