Well, sure you buy an ultra fast lens to use it at ultra fast speeds, but sometimes you just want more DOF, and it would be nice if the 50L was as sharp as the 50mm 1.8(a $100 plastic lens) at more mundane fstops. but perhaps choices in lens design have to be made and you can't have both. If it was the equal or close to the 1.8 at these smaller fstops i would have bought one years ago. every now and again i still think about it though. sharpness isn't the end all for me anyway. this most recent time the thought of buying the 50L was in my mind i bought a sigma 35 1.4, so who knows? maybe i won't like it, and i'll be back to thoughts of the 50L. (i'd like to see the 35L v2, but i'm afraid of the expected price)
also, i know what you mean about a camera bringing a lens to life. before i got my 5dmk3 i rarely used my 300mm f2.8. it was OOF so often and just never really did it for me. sure when it hit it was great but i just couldn't count on it. as a part time wedding photog i need to know my gear will work. that's job #1. now, with the mk3, i can use it like it was meant to be used. it's just amazing.
BTW- i've seen some very good low light AF tracking photos on the net from the 5dmk3 and 50L so it might just be up to the task. Personally, unless you think you can figure out in a day or 2 whether or not the 50 is right for you i'd buy in it instead of renting. the resale is high enough that you could probably save money just using it for a month, or a year and selling it if it's not for you. my 2 cents and my wife will tell you that's about how much it's revalue is