Haven't seen this brought up yet, maybe it has been. I think the Thieves Guild and the Dark Brotherhood main quest "areas" should be solo zones. I don't know how many times I've been caught trying to sneak because another person attacked a guard or NPC right by me. There was a few times in the Thieve's Guild main quest where I'd walk through the door and there was someone else running towards me with a guard in tow. With the Dark Brotherhood I am seeing that guards in some areas have been murdered but their bodies already disappeared and low and behold they spawn again right in the middle of me.
Just a though. There really is now benefit to seeing other players in these instances.