I haven't used it yet and myself questioned how to use the spell best (but without any moral regrets). The late champions are of little or no use to me - and the spell should be useful.
But even though I make my Sov a mage with two schools from start - it's only late, approx level 15+, that she has all talents I want and could use a new spell-line from Steal Spirit. I tried the spell on Bard henchmen but only get the henchmen-line (which only has one ability per magic school), but if the Henchman had Fire 2 then my Sov would get Henchmen-Fire-2. When trying out Steal Spirit on a Hero with proper Fire 2, my Sov would only get Fire 1. (This is all working as intended I think.)
Mage Sov
One option for getting use of Steal Spirit /when playing a mage/ is to skip one magic school when creating your own Sov. Those two points (out of six) instead goes to Talents (e.g. Brilliant, Might) or to Equipment (e.g. Procipinee's Crown, Ereog's Token) and when your 2nd to 4th Hero arrives your Sov sacrifices him to get the wanted school. The plus side is 2 more starting points, the drawback is your mage Sov has to wait with one Magic School, mening no use at all from it at the initial important gameplay. Also the first Champ is needed for a secondary army, as well as the second Champion if I'm playing on a big map and using Beastmaster (taming beasts to send back to Champions armies). It's a painful delay of magic IMHO and I'm still debating wether I should try it or not.
Might Sov
As a might Sov, I don't think the first spells from Fire1/Water1/etc1 are terribly useful in combat. Your Sov will mostly do more damage by attacking. The single exception I can think of is Death 1 (if Empire), giving you Curse (=foe get defense lowered to 0) and a possible huge bonus vs some hard monsters. If you are prepared to place some more points in the magic, you could consider:
- 1 or 2 additional point to get to Fire 2 for Flame Dart. If you have problem reaching your foe in the first round, this gives you a good ranged spell (2 x level +2/Fire shard damage). For 2 points you get the options Feedback (=interupt plus damage) and Focus (+25% damage) to cast the first combat round.
- 1 or 2 additional points to Life 2/3 for Heal and Shrink/Growth. Both of which I think could benefit a Warrior in certain situations.
- 2 additional points to Water 3 to get Spell Leech (and Mantle of Oceans for other casters). This would give you an interupt (same as Fire 3), something you don't have as a might Sov.
You will have to decide if it's worth skipping a few points from your usual talents (and the moral aspect), for the above. 
