NO. extra slots take to much space on servers. You must know data must be send between server and client to keep an up-to-date battlefield. As for spectator this means you would resevate an extra amount of Bandwith only for people that want to watch. I know what you mean and im upvoting this but the changes are low that servers will actually do this. Since it costs extra money to rent servers with higher bandwith.

The idea is that you can set the SpectatorMaxClients equal to the maximum amount of slots your server has (for rented servers that is), dedicated servers should indeed take note of the amount players connecting and the amount of bandwidth used.

At rented servers you could have a 40 slot server, but only have a 10 slot mission; so 30 slots are unused and might be used for spectating and/or administrating.

Especially for admins it might be useful to lower the mission slots by eg 5, and use them as "admin slots" (set SpectatorAdminOnly = true).

To be honest; I don't know what impact it has to the server if non-players are connected, but it should be less then playing clients.

True. But on the 10 slot missions on dedicated servers with 30 slots? there is areason for that. Arma 2 takes up tons of ram calcualting AI. Thats why people run headless clients next to Dedicatedserver.exe the Hclient calculates the AI while the server.exe sends data and makes everything work out. You must understand that 40 slot servers usually only have room for 25 to 30 people. Because that other 10 of players of bandwith Is reserved by either headles client and the server itself.
But yes its worth a shot defenetly. THey should allow people on BIS that have Bought arma be able to download a program that allows them to spectate and or play at the same time. THis would mean you can record and play at the same time either the mission or your own character.

Mass closing ancient tickets with no activity for > 12 months; assume fixed or too trivial.

If this issue is still relevant in current dev build, please re-post.