I agree that maybe the Duel gametype wasnt needed, but considering the number of people who supposedly play this game, even if we had assault you'd have a bunch of empty servers.
The thing about the Duel gametype is that is now sets a standard configuration of mutators etc used for 1v1. In Ut99 there was no such thing, and the result was the use of the no powerup mutator (which I preferred btw). This not only turned off the amp, but the belt and vials also. It's a pretty significant difference, but by Epic making this gametype (which probably didn't take much at all, other than a little refining of the DM/TDM gametype) they in effect set a standard for all to follow.
You're post seems like more of a complaint that Epic spent the time to make the Duel gametype, yet ignored Assault. The Duel gametype most likely isn't much more than an extension of TDM/DM, which they already had. Assault would have taken a lot more coding and bug testing, not to mention some support would have had to be in the editor for objectives etc. Apples and Oranges man, Apples and Oranges...