It's cool that you bring up Mass Effect, because that's exactly the game I was thinking of when you mention spectating aka backseat gaming. While it can be helpful and seemingly interesting to view the game from a second perspective, I avoid it with seething passion. When I'm gaming, I'm in the "zone". I get immersed and the last thing I want is whoever is sitting next to (or very creepily behind) me commenting or instructing. I don't even consider saying "here, you play for a bit". I like to keep my games "single player" if I can help it.
So now I'll go back to Mass Effect. I used to play that for long periods of time and my girlfriend would always watch. Now some people would say, "there's your problem." Wrong. She's a gamer too and can relate to the immersion one gets when playing a single player game. However, I can still hear "this is boring!" ringing in my ears. Some people just can't stand watching or being watched while playing games. It bores me to tears to watch Mass Effect, when I'm not the one holding the controller.
Ultimately, it's up to the individual. As for me, stop watching - backseat gaming is not my idea of fun.