i too miss essense. many reasons. to elaborate on GW's comments:
firstly as a means of limiting imbuing it was much better than mana maint. a one point per turn mana maint over the course of the game is absolutely huge, because the system is so coarsely grained. we have now created a situation where you'd be stupid to imbue anyone or summon anything (for more than a couple of turns) until you have a huge mana income (ie, late to mid game, when the game is almost won anyway). so you only imbue or summon long term as a whim because you can, rather than because it makes strategic sense. because mana accumulates without limit, every spell or imbue has whole-game repercussions, and this encourages people to be very miserly, which is sad (and if you are doing fine whilst spending lots of mana, it can only be because of poor ai and unbalanced casting costs). if someone blows all their mana in the early game, it's largely unfixable and could come back to haunt them as much as 10 hours later. of course, it's better than what we had before, when enchant slots were a triviality, and the costs of casting a spell were undone after 20 turns. a system that falls somewhere between the two would be better.
i love global mana. i think it would be better if it accumaled at a diminishing rate and then stabilised at a diferent level after 50 turns or something. or at least reduce some of the maint to 0.5 per turn (or whatever).
in the mean time however, it would really help to bring back essense, and spend it on imbuing (or enchanting, or creating fertile land or whatever). it made more sense in terms of the lore, but more importantly, it posed a more interesting tactical question. if you sacrifice your sov's spellcasting power to imbue others, you have to choose between being sauron or being the middle earth wizards: one powerful caster, or lot of weak ones all over the map? individual power vs. flexibility? whereas with mana maint you choose between flexibility or just not being able to use magic as often.
purely from a game balance view, magic weilding sovereigns should be as big a part of the game as combat sovs. however, a combat sov needs three stats to be effective while a magic sov only needs one. if this imbalance is not visible yet, it is only because the spell formulas are not balanced at present. once there are stat based damage spells competetive in the late game then INT will be a shoe in.
personally i'd do it like this:
int: limits what level spells a character (any of them) can cast, gives an admin bonus to income and determines spell resistance
essense: increases spell damage, spent on imbuing, enchanting items or creating fertile land.