I would like to hear your thoughts on this subject, but I think the Joker makes a kick-butt anti-hero when he's utilized that way, in a pseudo sense, in the comics. I'm probably talking pre-new 52, but during the rare occassions when the Clown Prince of Crime teams up with Batman or actually takes out another supervillain with a clever prank weapon and a one-liner, that is entertaining. I guess it could get old if it's overused, but I really think what keeps the Joker fresh are good stories and smart writing, not mass murder, unless it's just funny. Emperor Joker was a pretty good storyline, for instance, but it wasn't realistic at all, just funny, with a chilling look into the clown's obsession with Batman. Batman and Robin Must Die made Joker look like Professor Moriarty to Batman's Sherlock Holmes and presented us with one of those "even bad guys have their standards" moments. I like those "even bad guys have their standards" moments, and I think the Joker was famous for them once upon a time, with quips and comments about never stooping as low as a white-collar criminal and an obsessive need to take out bad guys he considered beneath him, like mafia bosses and drug dealers, if they were stupid enough to come to him for help. I think that is classic Joker and maybe what sets him apart from the rest of the DC super villains.
Also, the Joker was actually funny, once upon a time, opting for murder in the style of cartoon violence. Joker was sort of a proto-Mask character, only he used dark ironic comedy that depended on an impeccable sense of timing.
Sure, the Joker could probably never be utilized as a true anti-hero, but he could definitely play that role as part of a fun prank. The character is famous for being dark, intelligent, unpredictable, and willing to do almost anything for a laugh. The time in one of the Justice League cartoons where Joker was kidnapped by aliens and had to give a comedy performance was a funny concept, and, hey, no one got murdered or maimed. It was still a fun story, though.
I think the writers realized that Joker's gags were funny. That may be why they added some of the slapstick to Harley Quinn's performance. I think I see most of the "good guy" roles that could have been filled by the classic Joker being taken up by Harley--hero / villain team-ups and such. I don't think Harley doing those things has the same kind of impact, though, as Joker would have. For example, it's almost unremarkable when Harley agrees to work with Batman, but it's like "what just happened?" when Batman and the Joker work together.