A lot of my weekday riding is done on 2 roads that are extremely popular with cyclists and are somewhat busy. Some of you might characterize these roads as 2 lane "country roads," but they are major cut through streets for the suburban communities around here. Some of the cyclists are commuters, some are racers, some are serious recreational cyclists, and some are just out for a ride with the family, or are tourists (this route is on a map of popular bike rides in historic areas).
I always call out when passing people slower than me. I also often say hi to them, or "have a nice ride." About half of the riders passing me call out; these are usually the serious roadies/racers. What I dislike are the ones who say "on your left" when they are already right next to, or already passing me, not coming up on me. I usually can hear someone approaching me, but often, I kind of "jump" when another rider comes up on me, without warning. Usually, I ignore it, but sometimes, if I'm feeling cranky, I might say "Have a nice day," to them, in a kind of sarcastic voice. I know it's silly, but it makes me feel temporarily good.
I dislike riding on MUPs, and we don't have many here, either. When I have ridden on a path, I call out to every single person and vary what I say according to whether it's a kid, a slower rider, or a jogger/walker. But, it takes a lot of energy. The way I see it, it's like being an ambassador for polite cycling. If I am riding on a path, it's not for "training," and I'd like others to see that roadies aren't all spandex clad yahoos.