Arkhandus said:
I'm not sure how that would help any, when your first comment was that it makes them too hard to kill normally.....

And it's not like flasks of acid are all that hard to come by, even at low levels. And of course there are low-level arcane spells (Acid Splash and Melf's Acid Arrow) that deal acid damage. Any of those could be used to finish off a beaten-unconscious critter.
The difference between fast healing and regeneration being, that if you deal 100 damage to a creature with fast healing and some ability to survive negative hitpoints, then, the moment that ability is suppressed, the creature dies as normal (with the normal yadda yadda about -1 to -9 and so on). A regenerating creature further converts all damage to non-lethal damage (except those tryeps - like acid here - that "penetrate" the regeneration). So to kill a 100hp troll, for instance, even after you've dealt him more than 200 damage, you're still going to need to deal 101 acid damage as a minimum. You can try a coup de grace sometimes, but fundamentally, you're going to need a lot more effort to kill the beast.
That means that an acid flask won't be enough to kill an unconscious regenerating creature, not all the time, and if the creature has a few hitpoints, then you have a problem.
Further, regenerating creatures
can be killed by things like suffocation. I'm not sure if that's the flavor you're looking for, but it's something to keep in mind.
So a Fast Healing creature is easier to deal with because it won't rise again very quickly (with a regenerating creature it's usually just a matter of rounds) even with a special ability to avoid normal death, and secondly, once a creature has been dropped, it's not necessary to somehow do all that lethal damage.