Suppose you want to record 3 things: your microphone, game and Mumble. With multiple audio stream support, you can record what's in the input tab of pavucontrol (which contains the mic channel, and a monitor of your computer's sounds). Problem is, as you stated, is that recording the monitor input will grab every application (not good, for example, if you want to keep Skype open to catch any calls but not record it). This also mashes together all the audio meaning you won't be able to edit the game audio and Mumble separately.
What you can do, is run the command pactl load-module module-null-sink sink_name=game_out
This will create an output named "Null Output" in the output tab. Using pavucontrol, you can send the output of your game to the "Null Ouput", but you'll lose the sound. Running pactl load-module module-loopback source=app_out.monitor, however, will loop the Null Ouput back into the original output of the game, meaning you'll be able to hear your game again, but there will be a new input recor