Bayonets do take away from long range sighting, since the added weight and a few other factors work against the rifle.
which is another reason not to run around with them fixed all the time.
It affects it more at ranges at which the infantry would not be expected to hit targets with anyway. Even though the SVD and PKM both have bayonet lugs, I don't recall having seen either with one in use (I have seen a picture of a PKM with bayonet, but not in action).
Also, accounts from troops in Iraq said that mounting old timey 17" sword bayonets onto their W1200 shotguns (which has identical bayonet lugs to the M1897) made PoWs much more appreciative of their captors.
Again, psychological impact.