@Skeleton Jack: I think you have a pretty good handle on this skill. It is very powerful, but also very niche.

Primarily you use it to:
1) Heal zombies.
2) Heal characters with ridiculous HP pools that do not necessarily need a lot of AP (certain tanks).
3) Removing large amounts of banked AP from powerful enemies--this can become particularly relevant when using Blind effects.
4) Don't quote me on this, because I forget the exact mechanics of the skill, but if Fallow counts as a heal for Decaying enemies, you could do a lot of damage this way.

In the last full test I ran before Artificer update when live, I made a character that got some use out of Fallow. It was a Lone Wolf druid of sorts. This character ran all the Ranger/Geo gold skills and focused on dealing elemental damage. The character ran a staff for Overcharge and some Reaper fun. The backbone of the build consisted of poison damage enabled by Alchemist and a fair amount of Astrologer's Gaze. This character was not a Zombie but may have benefited from it. I also would have loved to run Pestilence and complete the mythical spear-caster-druid build, but I sadly never got one to drop. Note that this character was able to defeat the Void Dragon as more or less the only damage source in the party since the other two characters were nothing more than slightly varied support builds with heavy Witchcraft—so the concept has some merit.