From owning all these lenses, I could think of a few things that would make the 1.2's worth getting over the 1.4 and 1.8 versions (and this pertains to studios for the one person who said he was a studio only photographer).
1) The spectrial highlights in the out of focus areas look very unpleasing to the eye. This is true with every aspect of photography and with almost all aperatures on the lenses.
Meaning, with the 50 1.4 there is a halo effect in the little balls, where its bright in the centre it blends to dark then gets bright again for a ring around the light. With the 85 1.8 it does that to the extreme.. With the 50 1.8 it makes them a funny little non-orgainic shape (hexagon or what have you).
2) When shooting night photography, or anything where there is a really bright light source being captured on the sensor, if you shoot with high aperatures 5.6 and above, you'll get these star patterns, the most pleasing come from the 50 1.2 and 85 1.2, the 24L and 35L 1.4's also offer decently pleasing star patterns, the other lenses less so as you go down the list, the 50 1.8 being the least pleasing.
3) Sharpness wide open, if thats why you buy primes so you can get the artistic thin DOF then whats the point if your not going to get sharp images with thin DOF. The 50 1.2 has focusing issues right now, the 85 1.2 focuses slow, but if you get a good copy is sharp.. I've had a good copy of the 50 1.4, 1.8 and 85 1.8, they're good but you usually have to bump them all up to about 2.0 before you'd use them in a portrait setting..
4)I can' think of anything else right now, besides I have to go eat something, my post probably already doesn't make to much sense cus I'm hungry!
But don't get me wrong, the 50 1.8 boy I loved that little lens, and I got some good pictures with it, as well the 50 1.4 and 85 1.8 they all take stunning pictures and for anyone to say that you'd get better shots from a 1.2 is lying, at this point in the game it really depends on what the photographer can do with the lens, they're all awesome lenses.
Me I'm not an awesome photographer, so I have limits with all the lenses and I can tell what I like and don't like with all of them (85L to heavy) but there are some photographers out there that shoot with 85 1.8s and you'd assume they're using digital backs thats how stunning there photos are..
So no one lens is better, its just what suits your style, and if your style is night photography, and fooling with light bursts (in your portrait shots) then naturally the 1.2s are just designed to handle this better...
Take care,
Adam
P.S. I write long posts so my girlfriend doesn't bug me to help her with packing up the house, I'm trying to look busy