THM= dot/debuff mage
CON=whm/blm mage

Was always supposed to be this way but game was way unbalanced in the beginning. THM was never meant to be a 'nuker' mage or the main healer. Instead, it is supposed to whittle down a mob over time. I've played around with my THM post patch and it is still a pretty powerful mage, only now you get to do what it was designed to do. Lastly, THM is still the best 'main tank healer'. Sacrifice spells still do better than cures to keep tank alive overall and THM has the absorb def/attack/etc spells that can add extra buffs to tank depending on needs.