The way I see it is this.

THM and CON can both heal and damage well enough for things like leve parties and grind parties and the lower NMs, right? They're more flexible.

But for the more "hardcore" content you're going to want CON to stick to healer and THM to stick to debuffs/DoTs, such as on the HNMs and Dungeons.

To me this looks like a perfect set-up for the job system :P So it's probably only going to be like this until jobs come out.