I actually have no great need for a psu with all that potential power, but over the years i've had to get larger and larger power supplies so i figure what the heck just get the biggest one i can get and be done with it.
Also, According to the PC power and cooling site these supplies are more efficient then smaller ones, yeilding a power/energy cost savings over time. So the extra cost today will be made up over time with the energy savings.
There was a web site that i came across recently that shows how much power your system used based on the exact components you have. Depending on some variables you can choose mine would need up to 700w with 2 7900gtx's overclocked, x2 4400, 2 hard drives, case fans, 2gigs ram, etc.