To Pigeonpigeon, regarding your last paragraph: I want all channelers to start with all of their essence and all channelers start with equal essence (with a possible option equally available to all at setup to have extra essence - VERY costly). How you spend this incredibly vital, unique and limited resource should be what this game revolves around. Your point is valid, If channelers have all of the essence at the get go, how to keep them from being overpowered rather than growing into their power? Actually that is the answer - the essence is there but much of it is either untappable (they have to research in order to use essence that they already have) or like in MoM you limit how much they can channel per turn. Initially this would be a very tiny trickle. Again with research, the amount they can channel per turn goes up.
If they do make essence totally and irrevocably non-renewable, then that's the way I'd want it to be done. And I would be satisfied with it, I think. However, it might be harder to balance for different size maps and different length matches. And ultimately, I think essence would be just as important if you start out with a little with the ability to gain more little by little.
The major difference that I see is that with your method, people could down-right run out of essence and if things change to their disadvantage they'd be hosed. For example, people who invest their essence in other things could easily end up disadvantaged compared to people who hoard it (people who hoard it keep their options open and their channelers godlike). Late game could end up with a bunch of lame channelers who can't really do much anymore (no more essence to invest in anything, and due to said lack of essence their combat and probably spell abilities would be significantly reduced), and other channelers who are at the peak of their power. Yeah, the former would probably have more settlements or heroes or have some nifty spells going, but the channelers themselves would no longer be able to add or do anything. And if a game drags on, the situation would just degenerate.
Whereas if you can still get ahold of essence, it allows things to continue progressing even if the game does drag on. It allows channelers to continue to impact the state of affairs. It could also help alleviate the mopping up process when you know you've already won; once you've reached that point all that matters really is how annoying it is to finish things up, and being able to get and use more essence would make things significantly more interesting/fun, I think.
Another thing is, these are not completely mutually exclusive ideas. I like the idea that channelers start out with more essence than they can use until they put effort in utilize it, but that can be the case even if it is possible to get some more of it.