I usually try to leave a message if I know that I'll be gone for more than a day, but I have to admit that I am guilty of forgetting a couple times myself. Still, I agree that it does get annoying at times. Especially when people are trying to keep a story moving, or if a lot of people leave for several days at once without warning. If something happens like a power outage, a computer malfunction, or some sort of emergency, then I can understand, but otherwise, yeah... 

I'm going to keep working on getting better about it myself.