I've seen this question asked in many blogs I've read. We strive for the gold cap, but there's no way we really need all that gold... unless we want more gold. Even if you ignore the wails of the goblin gods and buy all the social vanity items there are and stand around in dalaran to make socials drool over your awesomeness, you'll come to a point where you got enough money to support you for a LONG time.
Gold cap was enough to support
Gevlon for more than 4 months and it just took him 100k~ Just don't forget that he paid 5k / week to the guild.
Since I'm neither a hard-core raider nor a vanity-loving social, I don't really need the gold. I could probably survive on my current stash until I lose interest in WoW. Still, I keep on going. I take risks and my gold amount fluctuate.
To answer the question, I have to think about what I've learned. What did I actually gain on my market activities?
* My epic flying mount.
I was actually really broke in TBC. I got my epic ground mount right before the launch of Wrath of the Lich King. It was a gift from a good friend of mine. I paid about half of the gold for my normal flying mount and got the rest from friends.
My epic flying was the first mount I actually paid for, all by myself(Normal ground mount was 100g when I reached level 40, that was a LOT of gold back then).
* Market knowledge
Say what you want, but this has certainly been something I've learned from. Sure, I knew the concepts of Supply and demand before I started using the Auction house, but not how to use information with Supply and demand to gain gold. The term of opportunity cost however was quite new. In my head, I already knew the principle, but I wouldn't obey it. In TBC, farmed herbs were free.
Having a developed market like the Auction House in World of Warcraft with spikes and drops is just what I needed to get a good view. Read all the theory you want, but a living example is always going to be needed to fully understand the theory.
* Boasting.
I have to admit, deep inside of me, there's a social. This being cares about the opinions of others, he likes attention and boasting is a must.
Slipping details to Real-life friends like: "Well, last night was a good night. I got about 8k gold." is something that happens.
* Freedom.
Freedom to buy what I want, gear my new level 80 mage up, do what I want. Freedom to be rid of the need to do dailies and farm to get what I want(Although, am I really free? I'll talk about that later)
These are the only reasons I do what I do. I'm reaching for the cap, but I'm not positive I'll reach it. I could if I wanted to, but of the reasons I mentioned, there's not one of them that requires me to reach gold cap... except maybe the boasting part.