Albert Einstein allegedly said
“Character power progression is the most powerful force in the universe. That's why we need frequent itemlvl reforms.”
Pablo Picasso said
“Art is a lie that makes us realize truth, at least the truth that is given us to understand. The artist must know the manner whereby to convince others of the truthfulness of his lies.”
Good games, just like art, lie to their players. They make them accept rules, which constrain them and thus create fun. If you want to have fun from playing a game you want to be lied to. If you don't understand this stop reading right now and feel lucky!
In Ghostcrawler's favor, I'd like to make one thing very clear: exponential CPP is not an aberration. It is the most natural way of increasing character power. If you gain 10 health while you have 20 health, you get three times as strong. If you gain 10 health while you have 1000 health, you gain almost nothing. Linear CPP becomes weaker over time! To grow stronger at a constant rate, you would need to gain e.g. 10% of health with every 'level'. That is +2 while you have 20 health, and +100 while you have 1000 health. Only exponential CPP makes you stronger at a constant rate!
The real question is not whether a MMO should have exponential CPP. All CPP should always be exponential unless you deliberately want diminishing returns for some reason. The question is whether a MMO should give +10% power with every 'level' or rather +1%.
What is important to understand is that Ghostcrawler's real reason for continuing the very strong exponential CPP in WoW is not that players need to see big numbers get bigger this fast to want to play the game. The big numbers during endgame are irrelevant for most players participating in that very endgame. Most of them never look at their numbers, or only at their dps in relation to what dps other players do.
I have always been very happy to gain a new item, even if it was just very slightly better. And, actually, the measurable impact on my character's performance from updating one single item was never noticeable in the first place!
Players absolutely care about even the smallest optimizations. Try raiding without reforging - you will likely find out that your drop in performance is not even measurable! Items are a strong incentive for many reasons. The fact that they make your character not just little bit stronger but a little more bit stronger is really irrelevant. Players wouldn't have a problem with much lower percentage increases once they came to accept this as the new status quo.
And even if there was any illusion (lie) left that players believed, Ghostcrawler just mercilessly revealed the truth to them with his blog post.
The real reason Ghostcrawler wants a very strong CPP is control. In his blog post he wrote:
“Such negligible increases can drive players to do some weird things, such as skipping over tiers of gear or entire levels of content. This is particularly relevant when we’re talking about a new expansion. We don’t want level-85 players to have a reasonable shot at level-90 dungeons and raids (or PvP opponents) just because that content is balanced for gear that isn’t much better than what the level-85 players have.“
Ghostcrawler wants to have control over what content you do with what items. So that he can do a better job at balancing. And I think he is doing the game a great disservice here!
In my opinion, the game would be fun if you could run a lvl90 raid with lvl85 items. Players would still run the lvl90 raids, because the items are better. But players would also enjoy running lvl85 raids for lvl85 items every now and then. In fact, this is exactly one of the things WoW needs right now!
Perfect balancing for a specific itemlvl is unnecessary - a virtual world balances itself in many ways. Great players should be able to run lvl90 raids with lvl80 items and less ambitious players should be able to run a lvl80 raid with lvl85 items to gain items that still improve their character for lvl90 raids. There's just nothing wrong with that. Nothing! Nada! Niente!