Hmm lemme think for a moment....
still thinking....
Why are we discussing this? Of course Discord will still be causing chaos! Just because he's with the namby pamby pony "good guys" now doesn't mean he won't cause some good ol' chaos. He may even teach Pinkie Pie a thing or two, or maybe its the other way around...
Besides if anything, Ponyville and Equestria need a lot more chaos. They can barely handle a basic disaster:
http://www.youtube.com/watch?feature=player_detailpage&v=EChAd-oaL_g
Pinkie and the towns reaction to Cerberus is priceless, mirroring what happened with Luna in Luna Eclipsed. What's more surprising though is that Twilight, instead of being the rational pragmatic person she is, takes the Rainbow Dash route and tries to zap the 3 headed dog to submission. No police, no royal guards, no organized evacuation, no planning at all for the disaster. Lo and behold though, its neither the brains, wits and magic of Twilight nor the guts, brash and brawn of Rainbow that save the day, but rather the diplomacy and care of Fluttershy.
How does the leader of the Mane 6, Twilight Sparkle react to disasters? Well its normally "all goes according to the script plan otherwise she often fumbles and ends up like this:
Do you hear that? The little orange bird is saying "cuckoo cuckoo." Let's hope those pair of wings are worth the hype.
Of course this little incident tops off all the disasters that struck Ponyville in my book:
Bunny stampede. That says it all. A friggin bunny stampede puts Ponyville into a state of disaster and panic. Oddly enough the person who again is best able to handle the situation appears to be Fluttershy, who by the way helped save the day and resolve more than just several crises. Celestia seems to understand that and lo and behold, its not the Mane 6 nor the Elements of Harmony, but Fluttershy that manages to turn Discord "good." That's a lot to be said about Fluttershy for what its worth.
But back on topic, Ponyville and Equestria as a whole consistently demonstrate they're rather poor at managing crises, always relying on hero(es) to come save the day. They're also seriously too wound up and lack exposure to new ideas and the world outside. Having Discord around causing "chaos" would not only possibly help better prepare them for future disasters, he could really help lighten the mood and open Equestria up a lot more. Sure Discord ruled over Equestria for some time, but I don't honestly think he deliberately killed anyone in that time...to my knowledge that is .
But if we accept Pinkie Pie to be Pinkie Pie, then Discord will be Discord, in a "good" way. If Discord is to be an integral part of the show in the future you simply cannot expct him to NOT cause chaos. He's an ally now and as long as the show remains PG 13, he won't stray too far from Celestia and the Mane 6.
Besides how many characters you know can top his swag?