Not mil, but I know and have trained or trained-with, plenty. We talk. But they are not on this forum: Essentially everyone hates even burst fire, wants semi-auto for all carbine work. It's all aimed, and even "suppressing" fire is just a number of individual aimed shots at a vague or invisible target.
But.
The MGs break (okay, usually just suffer stoppages that take a while to clear) often enough, and it's easy to imagine worse issues in a more protracted peer fight, that they want all rifles to be ABLE to go full auto. Then the team/squad leader can point at someone and say "you are the machine gun," to someone who can handle it, and he moves to a useful position, acts as their base of fire even though only an auto-rifle. Some units that do this have a TTP for anyone nearby and able to donating a few mags as they pass by on the assault, because a carbine loadout gets expended quick as an auto-rifle.
I know people who have done this in combat, have sometimes interesting stories of what happens when you stress test your carbine. It also is the purported (though pretty reliably) reason for the heavier SOCOM profile (but for 203 cuts, heavy all the way back to the receiver), to give a bit more mass for the relatively more common usage of the carbine as an ad-hoc auto-rifle.