The problem with this is whether you are racing or spectating you put the same strain on the server.Bots also but to a lesser degree. So a server thats already at its peak (lets say 16 players), cannot accommodate additional afk players without lag. If its a popular server, you basically need to get out to make space for someone else to race. Or the people actually racing are forced to run a even smaller grid.
Bots are currently automatically replaced by players (if someone joins) whilst game is in 'lobby'