Military shooters, I kind of like the two weapon set-up. I usually pick a good rapid-fire weapon and a good long-range weapon... when you need to use something different, the game isn't shy about giving you one.
But sci-fi shooters... I hate, hate, hate, hate, hate, hate the two weapon system. Sci-fi games should be about cool fucking weapons. We don't watch Star Wars and Star Trek for the military tactics, it's about the cool gadgets. But the problem with sci-fi weapons is that you don't always know how to use them tactically, so playing around with them in less critical periods is essential... and if you're playing Gears Of War, you don't really have the luxury of dragging around equipment you don't know how to use yet.