I had the honor of getting asked once, by a really fine band that tours Europe and has backed up Gregg Allman. A few of the guys from our band had driven to a nearby city to see them play.
Our band has a superb young guitarist. They had learned of him from their manager, and they wanted him up there for a couple of blues numbers. After getting him up on stage and talking with him for a couple of minutes, they called out to see if his bassist was in the house. I waived them off a couple of times, but they insisted that he have his own bassist for his numbers, so I went up. The tunes were standards, and the bassist for the featured band was ten light years ahead of me, so I took it as an honor and a very nice courtesy. I think they were mostly wanting to make the young guitarist feel more comfortable in strange surroundings by having me nearby.
I played my two, thanked them profusely, stayed for the rest of the show, and then thanked the bassist again. I even helped him load out some of his gear.
So, I go along with the 'treat it as an honor and don't abuse it' crowd.