I admit that I have flaked out on a few RPs. But I try my best to send a message to the person to at least let them know why. Sometimes I find that I have too many RPs going at once and when I get home from work at night I don't have the time to make a decent post for each of them. Personally, I feel that it is better to stop a game than try to keep it going with half-assed posts. It's not fair to the other person to do that. 

Sometimes I spend a lot of time developing a plot with someone and everything is going great and then after a few posts it turns out that in spite of us agreeing on the plot and creating a wonderful concept, their playing style is just doesn't mesh with mine. It's no one's fault, but it happens more often then you might think. Even then, I try to at least give the person a polite message telling them I don't wish to continue.

I agree that it's frustrating when people disappear randomly. I'd rather someone tell me that I'm a shitty RPer and that they don't want to play with me anymore than to just sit and wonder wtf happened.