The point would be having the weapon handy. It's not like a barreled action in an EBR goes in like an upper on an AR. It takes time to settle into the stock.How about another barreled action? Like running a NM prepped 22in with a scope for the long distance stuff, and bringing a 16in barreled action with a red dot along in the ruck for the up close stuff. Not as heavy as a separate rifle and runs from the same ammo supply as the 22in barreled action. Just need time to swap the actions. And a sidearm to transition to if you have a malfunction on the rifle.
If you're using an AR platform rifle, it's not like the mags are heavy. You could have 1 in the gun, and 3-4 in a pouch, and your mag pouch would weigh less than 4 lbs.
The ammo interchangeability discussion so far ITT is pretty meaningless. You're going to be carrying your ammo in mags, and whether they're UMP mags, Uzi mags, MP5 mags, AR mags, or Rem 700 mags, they're mags that aren't going to fit in any gun other than your secondary rifle/smg/whatever.