Thanks Ryan85 and awinphoto for your awesome replies. Correct if I'm wrong, if I read your responses correctly, the f1.2 does not focus fast enough to keep up with even normal-speed actions, let alone fast dancing. That was my main concern. It would make a dreamy still portrait lens, though, when we pose the groom and bride after the guests have all gone.
I would say that you are correct... I shoot studio and on-location portraiture and tried many lenses... If you've ever played with the 50 1.2, i would say it's kinda the same issue... focusing is too slow and too unreliable... and to it's defense, 1.2 is shallow... extremely shallow, so it's a lot of glass to move and not something to take lightly... So when it does nail the focus, it's very good... but, it can miss, and miss bad, especially in bad light... To me, i couldn't risk spending $2k on a lens that was a crap shoot if i would get a shot or not... the 1.8 was a lot more reliable, and fast and extremely good quality of imagery.