The only perk that would need such upgrade is firebug, but Id rather see a new flamethrower with more ammo than an ammo upgrade. Yes grenade launchers dont have that much ammo but COME ON its supposed to be used on big crowds not to shoot every clot that comes by. And besides, you can carry 2 grenadelaunchers, that should be enough, if not you are using them all wrong.
However, if you buy a package of ammo instead of a secondary weapon, then I suppose its ok. Like, instead of having 2 grenade launchers you could have just one with more ammo. That means the box of ammo should weight more than 2 blocks tho.