Overviews are NOT required for MP missions. Why? Because you don't see them. The MP interface doesn't show overviews.
Briefings ARE required. Even if you use simple text explaining the mission with links to markers on the map.
Like for a CTF map you should at least have markers for:
East Respawn
West Respawn
East Flag
West Flag
You should also have links to these markers in your briefing so the player can find them quickly.
A storyline is not required for MP missions OTHER than Co-op missions. These are just like SP missions without an overview.
As for in-mission cutscenes, DON'T USE THEM, or if you really, really have to, then make them VERY short. Otherwise all you do is annoy everybody that plays your mission.
Remember, online players are there for gaming, not the watching.