Fast Healing doesn't even offer much help in keeping us topped off between fights; For my (soon to be) level 20 Barbarian, a single past life would be like quaffing a single CSW potion once a minute. That's nice, in a very minor sort of way, but it doesn't actually change much: if I'm slightly hurt then I'll be chain drinking CSW potions to top off, and if I'm moderately or seriously hurt and there's no help forthcoming from team-mates or hirelings then it's time to break out the Silver Flame potions. At best, this would save me a little coin on one of those CSW potions, infrequently. Even triple-stacked, which presumably would represent a great deal of work, it wouldn't be enough of a top-up heal to save me a Silver Flame potion, and I'd just drink three CSWs before standing around for up to a minute waiting for this to proc again.
In combat, it is a potential life saver... but the stars would have to do some serious aligning for that potential to be realised. It has to come ready at the right time, it has to make a difference with a low value heal (90 odd points at level 20 ain't nothing but it ain't big, either), it has to not simply result in another heal overhealing, and all of this has to come about and tip a fight basically through lucky timing. It's too haphazard for such a marginal return.
All this is compounded by the fact that all epic past life passive benefits are defensive / survivability benefits anyway, which makes going for performance-enhancing actives all the more attractive for balance. Would I really want to spend three epic TR's worth of time and effort simply becoming more durable when I could become both more durable, and more effective as well? Fast Healing would need to a whole lot better than it is just to make that question tricky.