I muzzle 5 of my 6 mares. All of the muzzled horses are super easy keepers. Right now, they would spend NO time on the spring pasture if it wasn't for their muzzles. As it is, they're only spending a couple of hours every couple of days. I like that when they're on the pasture (vs. the dry lot), they are MOVING. The muzzles make them move even more.
Even during the height of the summer, they will all (but the easy keeper) where muzzles, they'll just get to be out there for more hours per day.
Additionally, we only have 1.25 acres of pasture right now, so muzzling keeps the pasture from being chewed down too fast. Of course, I supplement with plenty of hay.