Not just because she's inherently cool, but because she's very effective as a villain for a character who's hard to do good villains for. Byrne brought her back briefly to fight Cassie and established that she's a wandering spirit, something you can't truly kill, only contain; this is very useful for Diana, who unlike most capes has no reason not to just kill off a monster as blatantly monstrous as Decay by the end of the arc if it's at all possible. Plus she's all about collateral damage, which is one of the few things Diana's not that well-equipped to deal with (as opposed to a Lantern, or even Clark with his superbreath and heat vision), which makes for a good challenging fight.