I totally agree with that. However, I was talking about a pure caster mage - one who barely gets his BD stack to attack and hammers the enemy with spells. In such case you will kill with spells and that does not generate any mana back.

If you do have a nice stack of BD's, they are great for mopping up weakened enemy units and in such case, Transmutation is really nice. In that particular game I was referring to, I had only 1 available, and he alone wasn't doing much, so I had to compensate with Concentration rather than Transmutation.

Whereas the Mind skill, Holy Anger, pays off just from inflicting damage (any of the troops that can dodge, you get no mana from if they do) on demons and undead.
So a multi-attack (like Sea Dogs or Cerberus vs. 3 troops) can pay you 9 mana just for MAKING the attack, if you have the skill maxed.
Then your NEXT troop (Phantom of one of those two?) comes along hits a trio nearby, and there's another 9 mana... and if one of the stacks dies, NOW the Transmute kicks in.

If you're not going for high-score or high-level, learning II and III aren't particularly good actually unless you're a Paladin. At best you'll do slightly better than break-even by the end of the game, but most of the game will be harder.

I agree that for an expert trying for a high score Learning is a critical skill, but for a new player just messing around? Don't bother.