You buy the 50 1.2L for the buildquality more so than the extra 1/3 stop.
Although on some cameras it can been the difference between going from native ISO's into expanded ones, or getting that extra bit shutter speed out of it.
But again, mainly its build quality and smooth bokeh even when stopped down is what you get it for.
The 1.4 will do everything it does apart from the weather sealing, but just not as well - like it says on the tin, luxury
Canon EOS 5DmkII + BG-E6 + Canon EOS 40D + BG-E2N + Canon EOS 33
Canon 17-40 F/4L
USM + Canon 24-70 F/2.8L
USM + Canon 28mm F/1.8 USM + Canon 70-200 F/2.8L
USM IS + Canon 85mm F/1.8 USM
Canon Speedlite 580EX II + Canon Speedlite 540EZ + 2 x Nikon SB-80DX
Cactus V2s Wireless Trigger - 5 x
Cactus V2s Wireless Reciever