Well, it depends. Where do you draw the line really? I mean, I can agree with Grimm's point of view. If we've fought hard and long and that last hive is about to go down, I want to see it explode in a shower of alien goo so I can have something to write home about. If I'm on aliens and the marines have been turtling in their base for 30 minutes, hells yes I'd rather see them F4 than sit there welding everything that I just smashed.
But where do you find the exact distinction of when the game is clearly OVER and it's ok to quit? Maybe they've taken down two hives, but I've got fifty res and cargo's about to go back up and someone else has enough for an onos. Or that game we played last night, eh Owlzer? Anyone would have said it was all over for the marines, and rightly so, but we shoved on through and clutched victory from the jaws of defeat.
Worst examples of it that I've seen is people F4ing over tiny losses. OMG the marines took down double! Game over F4 F4 !!!
Thus the need for a hard and fast rule: If you join the game, you agree to play all the way through. If you do not intend to fight to the last man (or what have you), find another server.