I get the impression that there are many more people who would like observer than we realize.
When I tried 0 A.D. last year, in over 50% (rough guess) of the games I played, at least 1 or 2 people would request to be spectators.
Also in that game was the option for observers to join after the game had started.
Not realizing Jammy had started this post, I chatted with him and softcoder about this subject recently. They both mentioned the potential for network lag, which I understand is a valid concern.
I don't have much experience with network programming, but I have to ask if there's a way around that.
Observers don't need real-time data because they are not playing. If they have slow or bad connections, couldn't the packets be dropped? or queued, and then sent when their network connectivity reaches a certain acceptable level? And data being sent could likewise be dropped or queued? For observers, only chat messages, if any, really are the only thing that requires priority.
Allowing more opportunities for people to observe, combined with the ability to
join games already running may help attract new players.
So often players come to the lobby but a game is already running. Being able to join a running game may provide a "safe" place for new players to find out what it's like to play MegaGlest and how friendly the Coop games are.