I am not sure I like that trade but I'm going to give the logic. Given that Abreu will probably fossilize before he's no longer crewing 1b for the White Sox, if there's a hole that it is ok to open on this roster, it's trading one of Vaughn, Sheets, and Eloy. That's 3 guys covering DH and LF, and those 2 positions are generally fairly easy to fill with people from the minors or at the trade deadline, DH in particular you can fill pretty readily by just giving guys extra rest and playing your bench more. Furthermore, those 3 are perhaps the least flexible players on the roster as you can't exactly move them to CF or 3b if someone gets hurt; carrying all 3 actually creates some roster limits as 4 man bench with Vaughn and a backup catcher means you have 2 players left who need to be able to cover CF, RF, 3b, SS, and 2b on any given day. If one of those 2 spots is Engel, you almost need a 5 position-player bench to make sure you have depth if 2 IFs go out. Out of those 3, Vaughn almost certainly is the only one with value to a team like Arizona. In this case you are filling a hole with Marte by trading from the only position on the roster where you have too many players.