It’s was not as common as you think, only in certain circumstances was it allowed to use enemy weapons. Such as being cut off from supplies and being behind enemy lines where your own weapons would not be available or ammo of the type you needed was also scarce. It would be pretty stupid to put down the weapon you are used to using and what you have been trained on just because the other weapon maybe a bit better. If you run out of ammo on that system as well it’s hard to get ammo for it, as the only reliable way to get more is to scavenge from dead enemy soldiers. Even if you did grab an enemy’s weapon, after the battle you would usually get your old one back when you went back behind the lines.

The Allies and German weapons are very comparable. While the Germans may have had a good machine gun like the MG-34/42 and later the MP-44, the rest of the squads weapons were not that impressive. You would usually only have one machine gun and one sub machine gun in a common rifle squad, the rest would have Kar98Ks a dependable bolt action rifle. While the US may not have belt fed or large magazine capacity LMG it made up for it by having every soldier in the their squad having a semi or fully automatic weapon. M1 Garands, M1 Carbines, Thompson SMGs, Grease Guns, BARs, each man could put down a good amount of fire. While the the German squad had some good firepower most of the time if you were not an elite unit like Panzergrenadiers or an actual SS unit, you were not getting a bunch of automatic weapons. In a regular German squad you only had 1 MP-38/40, 1 MG34/42, and 7-8 Kar98Ks, with the ammo bearer or loader only had a pistol. The whole German squad’s tactics revolves around supporting and maneuvering the machine gun to make up for the lack of overall firepower each individual squad member had. While the US did not have a great suppressive weapon with the BAR, its overall squad’s tactics and firepower helped in its suppressive duties.

In summary usually troops would not use enemy weapons unless in dire situations or certain circumstances required them to use weapons. While some German weapons were superior to Allied weapons, it could be said vice versa for Allied weapons.

Bull, Stephen, et al. World War II Infantry Tactics: Squad and Platoon. Osprey, 2004.