I think the question of 1, 2, 3, or 40 is a matter of which game it is. For the game to work, Halo needs to prevent people from owning a map by having a shotgun, sniper, rocket launcher, and sword. For the game to work, Unreal Tournament needs to reward players for hanging onto a map advantage by affording them many options at their disposal. The weapons and maps and health bars demand it.
That said, I think more games could benefit from a hardpoint system, like mech games. A Rocket Launcher will probably take up your entire back (or both arms). A pistol will holster easily, if you have a holster. If you have an appropriate harness, you could shoulder 2 rifles easily. Heck, you could probably shoulder half a dozen, but you won't be able to switch weapons without dumping the ones in the way on the ground, and you're gonna be weighed down. Ammo is the next difficulty. Unless you have a pack or a special harness for that gun class, you're going to have to work to carry much more than an extra clip. And taking clips from a pack takes longer than from a clip harness on your thigh. Do any FPS games, particularly RPGs (even ones more like Halo single-player), use harness equipment? I can't think of any, and it would be pretty awesome. It'd be a must for an FPSRPG.