One of Call of Duty 4's best features is its encouragement of variation through the Barracks and experience points. You can rank up without ever using more than one weapon, but there are bonuses to be had in trying many weapons. I'm not usually an achievement-oriented gamer, but I try to get every accomplishment with every weapon in that game.
The most experimentation a typical RPG encourages is swapping between melee and ranged attacks. But they could use a model similar to CoD4's. Instead of making every weapon a separate skill path, you could just have weapon upgrades be unlocked through use and encourage (without demanding) the player to try something new from time to time.
That applies to single-player as well as multiplayer. But, now that I think about it, I'm surprised I've never seen a fantasy battleground game (ala LOTR: Conquest) with weapon and skill variety similar to Call of Duty 4. Instead of ammo, melee weapons could have durability stats and eventually break... forcing the player to either switch weapons or pick up whatever's lying on the ground where another player fell. It doesn't take much imagination to see how CoD4's multiplayer could be adapted to many settings and scenarios.
In any case, my point is that RPGs shouldn't always expect players to stick with a single weapon class. It's feasible for all skills (like Perks) to be gear-agnostic. The same player may prefer different weapons and gear for different scenarios. And, of course, variation can keep gameplay fresh and enjoyable.