I totally agree that Concentration is better for a pure caster mage.
In the game I'm doing now, I'm trying to see just how far a mage can go on a spellbook alone. I did some basic kiting (Debir-Scarlet-Bolo-Rusty-Verona-Umkas-Uzala-Elon-Sheterra-Debir) to Elon to get Moro (I need his freaking Splinter ASAP, so that I can start working on his purification straight away), got 1 Black dragon egg, a little cash and a few spells.
Well, suffice it to say, I'm still using that single Black Dragon (backed up by a ton of Black Knights that simply remain in the reserve and have never seen a battle yet) as my army and I've gotten Amelie close to lvl 41 - and she's still going and beating up armies and heroes like crazy
The bottom line - a pure caster mage does not utilize Transmute properly, as you need your troops to deliver the final blow (killed by a trap also works). As such, for that kind of hero, Concentration is much better, even if it's a little hard to get to it.