I'm not sure of many advantages, other than having an extra button or 2 available for the pinky finger. Otherwise it's basically just a matter of shifting your keyboard about an inch right or left.
I started as a WSAD user in 1999, but I turned EDSF in 2004. (And I'll argue the letters should be arranged in "up-down-left-right" pattern, not "up-left-down-right" :p )
More keys available on the left side. Easy to find by feel given the tick on the F, not to mention (but I will) that it's the natural location for your fingers to return to.
I'm more about the speed of things. I played a ton of twitchy 1v1 UT2k4 and had all weapons that would see real use mapped to a key right around my left hand. Also, jump on right click. Jumping was crucial to movement and the throw time for the spacebar had an (to me) noticeable effect.