The main reason of an ammo cap is to make people move. With limited resources you've got to relocate to get a good shot, you need to be sure of your ambush, you can't suppress indefinitely, hunkering down to endlessly plink at each other doesn't work....etc.
And...the ammo limit should try and reflect the difference in firepower between different types of weapons to add to the gameplay.
Ammo limits/box mags/etc...do nothing to limit hosing the field, that's up to the guy doing the shooting. The hosing is just briefer and comes to an end sooner.