I would potentially support some kind of consequence to prevent griefing/screwing the group, but it would have to be something that wouldn't take effect after (x) number of deaths, or something. For a couple examples:
1) Running a Waycrest, somewhere in the +7-9 range, and it's a smooth run on pace to +3 the key. We've finished the first three bosses and are going down the stairs to the Waycrests, when the tank and healer (who came together) abruptly drop group, leaving the rest of us in the lurch. That's the type of dick move- presumably premeditated- that absolutely deserves some sort of punishment.
2) My Paladin's first key as Holy: geared enough to do a +6-7, but decided to start with a +2 since I hadn't healed on that toon since Legion. So I joined a +2 group for Tol Dagor. I had more health than the tank iirc, but I thought "fine; I'm overgeared, and it's only a +2." And boy was he getting wailed on. Then I noticed that the mobs were bolstering...on a +2? As it turned out, the DPS who had the key was stoned and had listed his +6 as a +2. Oops. Well, myself and one of the DPS (a rogue I think) were the only ones that weren't undergeared for a +6, but we ended up slogging through it. (I try to run a key on every toon per week, so I didn't want to bail more than halfway through.) We wiped about ten times on the last boss (tyrannical week, and we had...trouble...with rotating Deadeye soaks and keeping people in the right spot, and I was still getting reacquainted with Pally healing), but everybody stuck through it and we ended up completing. Waaaaay after the timer of course. But- I would absolutely not have begrudged ANYbody for leaving that group