For me, it would depend on the airport and the WX. I've got zilch experience going into EWR, so I defer to the XJT guys here. If it's MEM and they give you an EDCT time, board and go. Odds are really good it's gonna be nixed by the time you get to the runway unless it's a WX issue. Those tend to hold up pretty well since MEM tends to go to hell in a handbasket during extreme WX events.
Oddly enough, there's actually GUIDANCE on this in our FOM. If it's gonna be less than 3 hours between push and wheels up, the CA and dispatcher have to coordinate a plan of attack and a hard time. If you hit that hard time, you're going back to the gate, period. I've played the "We're getting you out in the next 15 minute" game with ATC while holding short of the runway. After the fourth time of being told that and sitting out there for 2.5 hours, we went back to the gate. Sure enough, they didn't release MEM departures for another hour or more.
As for the not getting paid for blocking out for two hours. Rule #67 of ACARS ettiquette.....you always hit "taxi out and back" instead of "never left gate."
