I agree that the feature is realistic, but...
Does it really add anything to gameplay value?
Actually, it would add tremendously to gameplay value, especially as this is one of the things looking to be the most sorely lacking from civ5. While previous games had very clear methods of focusing on all-out military capacity or not (mobilization in civ3, and civics in civ4) there has been no suggestion of anything close to equivalent in civ5. The problem is, the economic aspect of a war needs to be as important if not more important than anything to do with micromanaging units. If units can be maintained and produced with hardly a misstep in other areas, spam becomes inevitable and the player gets a huge advantage using his units to tactical advantages over AIs.
Of course, to understand this you really have to understand a couple of other things first, so to back up:
-The human player should always have a massive advantage on actually moving units around, battling and so on, and this is amplified by 1upt and all the other designs implied in the civ5 combat engine (ranged attacks here too)
-If constant warfare/no significant costs to warfare is easy, then the human is encouraged to always be at war because due to tactical advantages it is the best way to make significant gains and progress in the game against one's opponent.
So to sum up - a mobilization factor of some sort, which the OP proposes, not perfectly fleshed out of course but it could be done in many different ways, would add greatly to the game. It would offer a measure of defense against trivial warfare and require true commitment on the part of both parties involved to all-out or total war. Causus belli and other diplomatic things are already not likely to be in civ5 at all, which I'm sure many of you know are in other strategy games, so without any diplomatic or tactical limitations on the player the economic aspect of warfare really could do with being included.
This is a bit like changing civics in Civ IV between Monotheism at peace and Theology at war. It made sense it terms of the economics, but was it fun? I didn't think so. And apparenly Firaxis didn't think so either, because it has been removed from the game.
If you are always in reserve mode in peace and active mode in war, then (to use Blizzard terminology) there is no "interesting choice" there... just a button that you have to push when you declare war. There's nothing fun or interesting about it.
Ah, I see part of the problem here, your statement clarifies what you are thinking. You are right in the sense that the player should not feel they have the most choice about the matter when they are at war. If you are at war, and especially if losing/doing poorly, you should feel the pressure to adopt whatever extreme measures you can to save yourself.
The most important part of the system, is to
encourage players NOT to go to war willy-nilly in the first place. If civ A wants to engage civ B in a serious war, and it involves both civilizations falling behind in economic matters and everything else, that should be a very strategic decision to be made. Other civs not at war make gains for choosing to stay at peace and not being involved. Again, this has been mostly lacking from previous games but at least was somewhat possible to understand in civ4 - you could switch civics before a war even, just to prepare for battle, even if it brought short term costs. But fundamentally the underlying system didn't have enough of an effect - in civ4, there was virtually no difference between being at peace and being at war with a distant enemy for diplomatic purposes, or almost being at war in general, if you wanted to keep peaceful civics. Unit maintenance was far too low, war weariness implemented poorly (only in enemy territory so you could slaughter AI stacks on a border) and there wasn't always a necessity to switch to war mode.
For the record, if I were to make changes in civ4 I actually wouldn't mess with civics or a new mobilization system too much, as imo the better fix would be to drastically alter how standing armies would work. I would make militia/city defense units, from archers on up, cheaper to maintain/build, and make offensive units have large increases in upkeep, to force a clear choice and investment when going to war.