I guess the practicality of it would come into it play. Also having to manage different barrel types etc could have issues. And you just know that gunners are going to take the light barrels when they should have heavies. Having the one barrel type keeps it simple and means they should get reasonably even use rather than one lot getting a licking and the rest bugger all. Carrying 1 + 3 spares for SF role I think is fine, and in the light role I'd prefer a slightly heavier standard barrel to any pro's a lightweight would offer. The different mods shown above are very interesting though and would like to see how the perform. I struggle to understand the angle the US Army took with the M-240(L7/MAG), the go to the effort of adding a F/Spt section to light platoons with M-240's and lightweight tripods only to then have short barrels on some them.I’ve often wondered why there haven’t been different barrels for light and SF role guns, or even light role guns having a light barrel for patrolling and a backup (or two) for heavier use.
Unless of course there are and I’ve never noticed.
We do it slightly differently (NZ Army), We binned our 5.56 C9's (FN Minimi with std barrel) for the 7.62 version as our section or fireteam LMG/LSW. Light, handy and is pretty much on par with the L7/MAG up to 600m. We still carry the MAG-58 when we need the extra range or additional firepower.