Several comments:
First - Yes, if there's an issue, it's less who was available last night than why were four guys unavailable, and that's a function of usage patterns over several games.
Second - De Los Santos is on the roster because they believe they need to have a long guy, in case a starter blows up early. With an eight-man bullpen, I don't think that's exactly unreasonable.
On the other hand, when you use a long man, you should let him pitch several innings (and actually bat! ), rather than yanking him for a pinch-hitter the first time his spot comes up. If you're going to use your "long man" for two innings, and then run four or five "short men" out there, you don't need, or want, a guy who's basically a starting pitcher in that role. So Austinfan's point is, I think, on the money - unless you intend to use a AAA starter as a true "long reliever" (which nobody much in MLB actually does any more), then don't put them in the major-league bullpen.
Of course, part of what's driving their choice of which AAA guys to bring up is 40-man roster considerations. They have AAA relievers they could be auditioning, but those guys mostly aren't on the 40-man (except Yack Rios), which means that to use one of them, they'd need to clear a roster spot, which usually means losing somebody, not just rotating somebody to AAA. They may get some roster relief soon, though; it looks like Cozens can be moved to the 60-day IL (foot surgery), and I'm thinking Arano's likely headed there as well (he's seeing Neal ElAttrache about his elbow - that's the Dodgers' orthopedic guy, and the successor to Frank Jobe). So we could be seeing Seth McGarry or J.D. Hammer before too long.