What I like about 11 tooth cog cassettes for racing is that the chain is more centralized on the cassette when your in the 14,15, and 16. I used to have one until I got my 10 speed bike. Now I can't afford another cassette at $150 as opposed to 30 with ultegra 9. I run a 12-25 whether its a flat crit, or a 25% hill climb.
one of my teammates once said, " If you have enough bal1s to throw on the 11, you better not pus$y out and go with the 11-23. 11-21 is where its at."