Sir, you have raised my ire.
You don't do it for BRAGGING RIGHTS!
that's RIDICULOUS!
What idiot goes through fifty (sixty) five grueling levels to say "DUDE I'm better than YOU!"
Just get 50 kills EVERY GAME and then you can brag.
Get your kill:death to 3:0 and then you can brag.
But just cause you PLAYED THE GAME LONGER does not mean you are BETTER.
You do it for REPLAY VALUE. People who BRAG usually GET OWNED sometime in their life, thus invalidating their FOOLISH CLAIMS.
AND THAT ICON IS REALLY FREAKIN' COOL, ALRIGHT?
P.S. On a serious note, the extra custom slots in COD5 are worth it. Why? Versatility, but usually only experienced players can use this to it's full extent.
P.P.S. IF YOU DON'T LIKE PRESTIGE, DON'T DO IT.
And in fact the challenges that have the name you are talking of in fact give you EXP, a valuable resource in a gameland filled with noobs, idiots, and cheaters.