Both in singleplayer world and in my private server, mobs do spawn from (especially from mob spawners) even if I am in spectator mode. Mobs already spawned while in survival won't despawn after I switch to spectator. Mobs can also see me as a spectator.

Also, I've build a creeper farm which works great when im alone, but as soon as I get my second spectator account there, spawn rates drop a lot.