Bayonets: Inertia in action
A brief history of the bayonet - Around the 17th century, someone figured out that a blade on the end of your muzzle-loading rifle allowed it to double as a pike in your infantry square (upon which infantry tactics had been based for about 2 millenia). The invention of spotted artillery and the machine gun circa WWI decisively ended the infantry square as a battle tactic (but NOT before a quarter of a million French Poilus were butchered). Therefor, the bayonet has not had a valid military purpose for almost a hundred years - not unlike close-order drill (but thats another debate).
However the military mind is trained to look backward, not forward. Therefor weapons are still designed with a bayonet mounting. Frankly I'm never going to believe any future war movies (Starship Troopers, et al) until I see the "Plasma Rifle in 40-watt range" with a bayonet mount. You KNOW the idiots at the top of the khaki pyramid will specify one in the purchase specs.
So the presense of the bayonet in endlessly rationalized. Guarding prisoners. Civil disturbances. Fill-in-the blank here. The official army line is that the presense of the bayonet on the rifle makes the individual soldier "more aggressive and more likely to finally close with the enemy".
But its all marsh gas. If you want to hang a bayonet on your smg, by all means go ahead.
I'll just pack the equivalent weight in ammo instead and be way ahead.
Yeah I have a bayonet for my Uzi and for my Swedish K and a few of my rifles. But they're more for conversation than for anything serious.