This thread is just repeating the same points over and over.

Leave because of an emergency or something you don't have control over?: You're fine, everyone that isn't a child will understand.
Leave because of something you knew prior or have complete control over?: You're most likely a jerk and a large waste of other's time.

Just be considerate and don't join something if you need to leave before it might be over. Sure, you could get a lucky fast run, or you could get a longer average run. Don't inconvenience others because you are impatient and greedy.