While I like the idea from a logical standpoint, it does actually make the game a little easier. I know it's not meant to be "DoomRL: the backpack management simulator", but inventory management is a huge part of the game.
If I could carry around a plasma rifle, bfg or even a bulked out ML/combat shotgun without their attendant ammo-slot, I would. Just quick switch to it when you want it, stock it up with ammo from the ground when needed. The extra versatility of carrying an off-spec weapon is balanced by having to give up 2 inventory slots if you want them to be convenient. You can just one slot it, but it's slow and fiddly (having to drop something, pick up ammo, reload, drop ammo, pick up original thing) and whilst this sounds strange, I think that's how it should be. Either sacrifice 2 slots, or deal with the hassles it causes. Your choice.
If I could just swith, load-off-the-ground, switch back, there would be no downside. One inventory slot for heaps of extra versatility for any character. Whether this is good or not is up for debate, but it would make the game a bit easier (frees up an inventory slow for one thing, would probably be quicker than the fiddly 1-slot reload procedure for another).
Compare it to Juggler's quick-switch multi-shotgun-in-backpack reloading. It's not that much harder and you won't use it as often. They're both fiddly, but the extra versatility they provide does have some drawbacks to them.
So I guess I'm against the idea, even if it's a great idea and makes sense, it does make the game easier and removes one of the only stumbling blocks that Doomguy has to stop him having a backpack full of weapons, rather than ammo for those weapons.