Reviving KOed units
Yes. It does this. You know this. Next.
Increasing the max hp of units
DE thrive on getting increased HP on their units, and even a little bit of HP means a whole lot. For example, a VM at 960 HP takes 4 hits from an unupgraded archer. But add just 5 HP for 965 HP, and suddenly that VM takes 5 hits! That extra AP to kill? Priceless. This is basically why people complain about unkillable monster VMs. :) To give your opponent even more grief, aim for >1080 HP VMs - those will survive 3 hits from a sworded archer (that's a scrolled hit!), and truly live up to their name of "monster VM.
This is not to say units other than VMs can't benefit from soul harvests - check out this Impaler. Without even a single piece of equipment, she's over 900 HP, which means it'll take 4 hits from an unupgraded archer to take her out. That's as good as a unit with a shield! And remember that the buff applies to all units on the board, so you could conceivably have your whole team effectively shielded.
In particular, this is a real boon to your priestesses, as you would normally never think to spend equipment on them. With souls harvests, your priestesses get a free ride and additional protection for free!
I can hear the outraged cries now. "Damage?!" You cry. "You mean that paltry 100 damage that gives even the puniest cleric the tickles?!"Yes, damage. Specifically, damage on 650 units, and especially against TF. Why?
Check out the HP of this spy. 550. It may not occur to you right now, but remember that every action in this game is governed by AP. 650 HP might as well be 800 HP for all that does - your 200 damage units still need 4 hits to KO, and your 300 damage units still need 3 hits.
But wait! After the soul bomb, spies have only 550 HP! That's 3 hits and 2 hits respectively! That's -1 AP, and that is huge! Also, keep in mind that the soul bomb has an area of effect - you can rack up a whole lot of -1 APs to kill in just 1 soul bomb!
Even outside of 650 HP units, 100 damage can come in handy in lots of situations - take for example, newly rezzed units. Apart from the cleric and upgraded medic (boo) freshly rezzed units have 100 HP or fewer - feel free to drop a bomb on them and then spawn a wraith to stomp for good measure! There's no hiding from the soul bomb. :) Soul bombs are also fantastic for taking out phantoms (you can get ridiculous HP buffs this way) and for dealing crystal damage (remember that they deal full gem assault tile bonus damage, which can be really significant especially when you get hit 2 crystals at the same tile, like on the tribe map or the staggered crystals in a field map).
Blobertson also reminded me that this piddling amount of damage? Also breaks bubbles, beer, TF medic link, stuff like that - it's particularly good against TF medic because the link target needs to get healed up before the link can be re-established (shades of matrix), whereas it's not that good against bubbles because soul bombs that don't do damage don't give bonus health. Still, it's a good option to look out for!
All of the above
But you know what the best thing about the soul bomb is? It's the fact that all of the above advantages aren't mutually exclusive! If you're lucky (or you plan ahead), you can have the soul bomb Rez your down unit AND bump it above 900/1080 damage AND damage a whole bunch of spies AND deal crystal damage with a cherry on top!
Now that? Is the perfect soul harvest. :)