There was some talk a while back about how "special" units would require some sort of mana expenditure. You can't simply make "paladins", you have to design a unit and then imbue it with some sort of power to make it special. Its safe to assume that the pinnacle of "elite" troops would all have some sort of magic investment. It doesn't matter what form that invesment takes, it could be magic weapons, magic buff spells or what have you.
Did any of the devs ever mention that special units would require some sort of mana expenditure? I have never seen such a statement... I've seen lots of speculation and hoping on our part about that, but to my knowledge (and I'm kind of obsessed with checking these forums ) there has been zero confirmation. Additionally, based on what we've been told so far, it looks like essence and mana are going to be two separate things, with essence being the much more valuable one. If that impression is right, then there is a huge difference between troops that require a mana expenditure vs. troops that require an essence expenditure.
I think that in general, essence will (or should) be something you invest in things that you intend to be permanent fixtures of your kingdom - like your channeler, heroes, settlements, items, miscellaneous bonuses to your kingdom, etc; while mana would be used for things like strengthening your military, casting spells, the kinds of things that are either one-time uses, constant upkeep or things that don't tend to last long (like individual soldiers).
The power we use to fund that investment comes from a finite source. This is what makes it an investment versus an expendature. Everything we have heard to date about "essense" and all that details how players will invest or hoard this power as they see fit. If you spend all of your power making a titanic killer stack, there is nothing stopping me from gathering up an equal amount of power and casting magic spells that nuke your stack from orbit. In that way "essense" or whatever you want to call it should be self balancing. Power spent in one way can be countered by power spent in another.
Well quite frankly, we don't know how finite or infinite essence will be. It may be something that increases with time, meaning that it would be no more finite than mana, or gold (but it would probably be scarcer). And you should look up 'investment' because your sentence using it doesn't really make sense ... An investment is merely an expenditure of resources to make a profit, or more generally, an appreciation in value. Whether you spend gold to train troops, or mana or essence, it is still (usually) an investment - you spend some resources to recieve a 'finished product' that is more valuable to you than the initial resources.
The rest of your paragraph, I agree with you. And actually, it just made me think a little harder about the subject. The example given a while back about use of essence was that one player could choose to dilute his channeler's essence into several settlements, kickstarting expansion and allowing for higher military/tech production, while another channeler might horde essence; and a conflict between the two would be balanced - large, advanced military with a weak channeler vs. small army but powerful channeler. In either case the investment of essence goes into something permanent - the channeler is obviously permanent because if you lose your channeler you lose the game; but even the investment into settlements is pretty permanent - even if some of your settlements are conquered, or razed, you could still take them back and rebuild - the land would still be habitable thanks to your past expenditure of essence to make it so.
But then, let's say you can invest essence directly into military units, resulting in really powerful soldiers. The major difference with this scenario is that the investment would not be into something permanent. In large-scale battles, you will almost always lose some troops in every battle, even if you have a huge advantage. This means that over time, your investment will be whittled away until all that essence is gone for good. To me, this would mean a third scenario to frogboy's above: a small army (about the same size as the essence horder's) of very powerful units, even more powerful than the army of the essence disperser's large, advanced military. In this situation, it doesn't balance out as simply; but rather, the player with the essence-imbued military obtains a short-term advantage in return for a long-term disadvantage.
At this point its hard to argue for one side or the other. We just don't have enough info.
That was kind of my point. We don't have enough information to go about making it seem like we know things we don't know. We can argue and discuss and hope all we want about what we want the game to be, but it just rubbed me wrong that you stated something that's never been even remotely confirmed as if it were.