Ladder ranking systems like ELO just don't work very well with small pools of players. Even if we had 100+ active players in the community, it still wouldn't be enough.
Observers are doable if you don't mind them counting against the 6 player limit. The hard part about implementing that would be hacking apart the MP lobby, which is something that had been discussed in the past in order to implement other proposed features, and we'd have to hack in special cases for observers in the code that determines the soft structure and vehicle caps per player (observers shouldn't count against that), and maybe not allow their chat messages to be sent to other players.
Although, I would say making a working game replay system would be a higher priority, which is a similar feature but doesn't limit you to only being able to watch games that you join as an observer before they start. BlackBox attempted one a long time ago but it had a lot of very, uh, interesting bugs.
