Well, it's really for multiplayer. I think it would be a nice option for the host to have. The host can choose not to have a timer at all.
Currently, a multiplayer game could last for days straight depending on the skill of the players.
I can't figure out what gave you the impression that I was talking about SP? The original author pointed out that this would mainly be for multiplayer use, and I didn't say anything to indicate I was talking about SP. You seem to dismiss me as talking about SP, solely because I don't fully agree with you.
I would like the option to host or join a game that I know will only last 2 hours (or whatever time the host wants to set).
Based on the game settings, you are normally able to fairly sucessfully estimate how long the game is going to last. So if you show some forethought, you'll be able to play a game within the time limits you have. Other than that, most other players are often understanding of the fact that you have to leave. Several times, I've ended games as either an agreed draw (locked teams) or as a winner (unlocked teams, all remaining players ally) because either my opponent(s) or myself had to leave. Sometimes, we did so simply because the game had reached a boring stalemate.
I still believe that having such a timer as an option would be useful, but I also still think its use will be limited. Most players simply won't want to constrain themselves time-wise, unless they share the host's time plight. Sure, you might be able to increase the usefulness of the timer by making it subject to in-game options (determined by the host or votes), but that would just open another can of worms.