I've used Essence bards since I started with RM2. (OK, I started with RM1 but never had bards around until after RM2 was out.) The main rationale actually came from the monk side, where there's a strong argument that the monk's abilities stem from their mental focus. So for balance, the bard goes from mentalism to essence.
I've also created a channeling bard, culture-specific. They're orcish bards who channel the power of the battle-spirit (dzos) for their songs. The limitation is, they only get half of their normal PP if there is no battle within range of them. So far, it hasn't been imbalanced either way.