I wish people would stop bringing this 'Realism' thing into this argument... the 2 weapon set up has absolutely nothing to do with realism and everything to do with game fluidity. Simply said, if you give the player more then 2 guns at any given time you're making the game a lot more difficult.
When you have 3 types of melee weapons or 8 types of hand guns getting to that one specific which would be the best for this situation is a trial in and of itself, exspecially when in the throes of heated combat.
In games like Half-Life, Doom, and Quake you'll often get 'item confused' as you try to quick switch to a specific gun and pull up something completely diffrent. You'll throw Grenades when you wanted to shoot rockets... Pull up a pea shooter pistol instead of a magnum... or worst of all, pull out High Damage AoE weapons in close combat.
this effect can be seen in any game that has a selection that has more then 2 primary tools...
Deus Ex, Terraria, MineCraft, Garry's Mod, Red Dead Redemption, Resident Evil 5, just to name a few.
This is so well known an effect that it has long since been a factor in fast paced competative First Person Shooting games from the Orginal Team Fortress to Counter Strike.
you want players to be shooting...
not juggling weapons...