I play with my wife as the cleric. When she tells me that she is oom and can,t heal my tank anymore, I click exodus. If I wait to see if my tank will prevail the usual result is a complete wipe - not good. Quite often the tank does get left behind, but that's better than all dieing. Of course, we are causual players, not involved in raids, or for that matter, any high level named characters.
The time it requires to get back to where we where is never a factor for us - it just repesents more loot and XP.
Our usual group consists of 2 level 66 Druids, a 66 cleric, a 67 Rangers (I know, he is supposed to die a lot, and he does) and a 63 enchanter. WE have fun and do pretty good as long as we stay out of the planes - haven't figures out how to handle those yet.
Oh, it also helps that we all have veterans reward complete rez, as well as the cleric having the AA for 100% rez. Still, I think it's best in casual play to succor when it looks like a wipe is inevitable - and I think most folks know when that is coming if they play with the sme group on a regular basis.