I think for long, fun, singleplayer campaigns carrying only two weapons is bullcrap.
It ends up forcing you to decide between the really fun, explosive, over the top weapons, and the safe, standard, all-around weapons you think you need and just prevents you having an ideal setup.
For example, in Borderlands, I got to carry around my badass electric pistol, my SMG, my Sniper and my Rocket Launcher. It was essentially perfect.
However, in a game like, say, Brink, I don't mind the two weapon thing. It keeps things neat and focussed with a heavy primary and a light secondary, and it's easy to get to a command post to switch out. In that case, it's a tactical decision you have to make, but if you make it wrong it's not going to make the whole game a tedious affair.