I was also sidetracked because of surprise visitors. It's really a bummer to me, unless just a drop-in. I think a phone call, a plan, an understanding that it's a good time - that is a courtesy that helps so much. Definitely should be so, if you are going to be accompanied by children, since they are going to require more from their hosts than adults would.
One thing that should be a given is that in a household in which people can't/don't eat the super-convenient foods, cooking needs to have time to happen. So even though the visit may not be AT mealtime, it still may be during a time cooking needed to be done, or the household's plans are put on hold. Also, if seated on someone else's couch during a surprise visit, why would your entertainment become your own phone? Then you remain the focal point for your hosts, but now they watch you NOT interacting.