Would it be possible to implement a feature where you'd queue up with your friend, and be able to select either "play" or "spectate" in the waiting room? That way you could spectate your friend's ranked game, considering most people don't play custom games (unless it's for practice or farming). I really do believe most people will want to spectate ranked games. See how their friends perform in games, and so on.
Also, without having tested the feature yet, is the spectate mode on a delay (1:30 minutes or something)?