But some people - many people, actually - DO save large portions of gold for periods far beyond what's realistic for purchasing/upgrading of certain things. Yes, units aren't individuals and do evolve over time, but they don't evolve over time in Civ. We've all had warriors sitting around for hundreds, thousands of years who later on we say "oh, I'll just upgrade this guy." In Civ, a unit goes from X to Y to Z in single, one turn gold fueled bursts, and oftentimes they'll go from X to Z directly, or even more extremely W to Z. Are they all exploiters too? Of course not - the things they're doing are just a parts of Civ.
These examples could all be given the "oh, they're not immersive" treatment but, frankly, they largely aren't, and few of the examples I cited are considered exploits. Many of them deserve to be judged by exactly the same standard you guys are holding against SP storing, but they aren't. Go figure, they're just part of Civ.
You mention "]I dont think anyone saves all their gold from the start of the game you spend it, make more, rinse and repeat" as if this exonerates the game of giving you the ability to do just that. Well, guess what - people DO save gold for extended, totally unrealistic periods of time. What's more, the fact that people are saving "gold" in periods before currency was even invented, saving it for hundreds of years, and then using it to instantly buy items in their cities... Sorry, at this point, you've already thrown reality/immersion out the window. But, it's been given a free pass because it makes sense from a gameplay perspective and there's some sort of historical phenomenon which is being hugely altered to suit the purposes of the game.
So why are we picking SP saving to be the one that "ruins immersion" and is an "exploit"? Why not prehistorical gold saving and building buying? Why not units that don't evolve over time but get instant upgrades? Why not bearskin wearing warriors being an effective garrison in an industrial aged city?
I've given a historical case for SP storing and you've even admitted there is some sense to it. SP storing - or, cultural storing, whatever you want to call it - has some reference to an actual historical phenomenon. It's a stretch to imagine it implemented as is, but as should be clearly evident from many of the examples given, the game is full of similar stretches that require similar suspensions of disbelief. Why are we picking on SP storing? Why is it an exploit, why does it ruin immersion, etc, and why not the above things? If held to the same standard, they should be. Obviously, they're not being held to the same standard.
Oh, and, just for the record:
http://forums.civfanatics.com/showthread.php?t=399722
This is a poll for how people actually use stored culture. You'll see the people who save it all from the get-go are quite the small minority.