In MP you get respawn and non-played players won't appear (depending on settings). You can also use the MP chat commands (sideChat, etc, both in the mission and for debugging purposes).
I only use the MP editor, mainly because it is generally more appropriate for what I'm doing and I can't be bothered moving the files from Missions to MPMissions folders all the time. Yes, preview is a bit slower, but that isn't so much of an issue for me compared to the other advantages.