The problem with that is that it's not interactive. If you fall of early because of lag or a mistake and it's a 10 minute fight, what are you now doing? Playing on your phone, alt+tabbing to something else, getting up and leaving the game entirely, or in the worst case scenario, the group calls for a wipe and restart because you now can't beat enrage and will fail in 8 minutes (granted this one can happen in some fights anyway if you're very tight on DPS already, but this would make it far worse).
All of those, frankly, suck. Telling people to stop playing the game for extended periods of time is flat out poor game design as you're encouraging people to leave the game to go do something else. You're not allowing them to learn from mistakes by getting up and trying it again, and you're excessively punishing the rest of the group by dramatically increasing the cost of a death to the party. If a healer dies, should the outcome of that really be to punish the other healer by forcing them to solo heal the entire thing?