I've had a long running debate with some folks I play with regarding concession. Some argue that is it is unsportsmanlike to concede a game if you are not doing very well. I contend however, that if it often more frustrating to sit there and be miserable if you know the outcome of the game; this is particularly true of longer games such as Catan with Cities and Knights.
Now, I'm not really talking about "just quitting" every time you don't get your way. And I'm certainly not talking about being rude, snide or otherwise contemptible in such cases. I've been on the other side of the fence too and it seem appropriate to "just call it" in many instances but some would rather drag it on.
Is there an acceptable middle ground or technique for dealing with these types of situations? Where one person has clearly won before they have actually won?