I've seen other species of hummers other than Anna's be very aggressive in guarding feeders in my yard, including Allen's and Black-chinned. The Anna's are definitely the most territorial, though, and manage to hold Feeder Guardian posts longer than any other species. While I do have Allen's all year around, there aren't that many in comparison to the numbers of Anna's, so it's almost always Anna's guarding the feeders.
I don't know if it's that the Anna's are here all year and breed during the coldest part of the year so they instinctively guard their food sources as much as possible given the competition - or that they are very aware that they've found themselves a bottomless source of nectar in a feeder, as you said.
I do have one Anna's now who is a bit of a gentleman; he sometimes allows lady Anna's to feed at "his" feeder without harassing them. He won't tolerate other males, though.
A month ago, I had a little female Black-chinned who guarded a Cape honeysuckle plant in my back yard for a few days. She didn't try to guard either of the nearby feeders, although she'd zip over to them if their Guardians were temporarily absent. But woe betide any other hummer that tried to feed from her Cape honeysuckle!
Otherwise, in my yard, sharing usually only happens in the very early morning and at twilight, when the birds are more concerned about tanking themselves up with "juice" and less about competitors. Although, I have to add that I do often see girl hummers kindly sharing a feeder at any time of day - they're usually wary but not aggressive towards each other. And, if a Guardian is away, even visiting male hummers will share in that brief time before the Guardian comes to drive them off.