Because this isn't chess, the matches really only take but a couple minutes and there's always a reasonably fair chance your opponent might slip up and cost himself a guaranteed victory.
I've wound up winning quite a few matches because my opponent made a mental error, miscounted his/her attack with a certain card, or misclicked while piling on the final round when I was pretty much assured defeat. (And had a couple where I choked it away, heh. )
It prevents the person you're facing from completing missions, it soils the taste of victory sort-of-speak, and I'm guessing you get more clintz/experience from wins then from forfeits. Not sure though.
This isn't chess. A match only takes a few minutes, even in ELO. If you know you've lost then why not just simply take your turn quickly? Not like it's gonna take long if you're not taking long to take your turn. If the game is already won I doubt your opponent is going to take long either.
People shouldn't be conducting missions in elo and DTs, it's just as bad etiquette expecting people to waste their time drawing out a loss as it is to forfeit and prevent a KO. Fight Club is perfectly fine for that.
I prefer it when people quit in elo, and if I mess up I'll leave.
The only people who really annoy me ae those who have lost and let the timer tick down almost fully before playing their final hand(s) in the hope you'll disconnect via network error or frustration.
Fight Club has no rules really so you could face any kind of hand and there's no point to it. There's no rewards for competing in it. That's why it's better to just compete in ELO and DTs and complete missions as you go. If you're aiming for a specific mission then fight club usually works best, but the only time you ever really need to complete a mission urgently is for LDs and sometimes those require you to play in ELO or DM anyhow.
Personally, when someone quits or forfeits on me after I've clearly won I don't feel like they're saving time really. It's not like matches take long as long as neither side is stalling. Rather, I feel like they're robbing me of an actual win.