"Are you really trying to tell me that all the progress in the form of levels and gear you get in a MMORPG is a function of your *skill* in playing that game, acquired by many hours of training? Balderdash!"
I think we need to differentiate between you the player, and the character that you are playing. In RPGs (MMO or otherwise), it is the character whose skill increases as he or she spends time training. "Look! I have learnt a new spell, and can now cast frostbolts at my enemies. My skill in magic has increased." This says nothing about player skill. Their skill in magic is still zero. Players are roleplaying a character. The person that is making "progress in the form of levels and gear you get" is the character, not the player.
In this roleplay, it is quite easy to accept that time spent by our characters in practicing their skills actually improves those skills. It just so happens that the time the character is active is equal to the time the player is playing him. The same is not true concerning money. We do not so easily accept that our character's in-game wealth should depend on the player's expenditure.
I (the character) acquire items by in-game activities such as killing kobolds and taking their candles. Then one day I wake up and find the Eternal Staff of Uber-Pwnness in my mailbox. How did it get there? What is the in-game activity that explains its arrival? If there is none, if I get it simply because the player playing me bought it in real life, I have broken out of the play I am in, the actor is suddenly jerked back from the story to the stage. The same thing happens to me if it appears in your mailbox instead of mine (and I become aware of it, for instance by seeing it on your back).
Why do we play MMORPGs? Everybody has their own reasons, and I'm sure Tobold realizes that the fantasy of killing the Lich King (and not the reality of pressing keys in front of a screen) is a reason for many people. In other words, we play for the story, more than the gameplay. Of course the gameplay isn't unimportant, but RPGs in general and WoW in particular have such great stories. Without the stories, we are just killing ten rats to collect 10 XP.
Each time we are jerked out of Azeroth and put back into our living rooms, our enjoyment of the game diminishes. There are plenty of times this happens, and it seems to be getting more frequent recently: the dungeon-finder teleport, frost-emblems that magically appear in your purse as soon as a boss dies or a "heroic" is completed, greed steeds, pandaren monks, Mr. Chilly, and even the older battleground teleports (though at least I can sort of believe that I'm being teleported there by the battlemaster I spoke to earlier), trading card items, and the Zhevra mount and its replacement, the X-53. It's hard enough to ignore these immersion-breaking elements. I don't want to see an increase in them.
I understand that for many people, its a game of collecting (gear, bosskills, gold, achievements, reputation, titles, pets, mounts, recipes), and the acting is not so important, and I can understand that they may not be so perturbed as me, as long as the thing they collect is not impinged by a real-money transaction. However, once it does impinge, many (not all) collectors are also perturbed by it.
For instance, mount or pet collectors who want the whole collection must now go out and spend real money on top of their game fee for the new RMT pets and mounts. I see many players on blogs saying "yeah, so what? These items don't affect the game. They are cosmetic. Blizzard would never introduce RMT items that matter in-game". What they mean is "yeah, so what? These items don't affect my game." They don't matter in-game if your game is gear-collecting or bosskill-collecting or title-collecting. But they do matter in-game if your game is pet-collecting. To a pet collector, dismissing them as "cosmetic" has a hollow ring.
Anyway. In an MMORPG, it is not surprising that a character should improve his skills as time passes and he practices those skills and trains them. In WoW the time a character spends and the time a player spends on this is the same (not so in EVE, where characters may continue to train when the player is offline). However if we allow players to acquire items in-game simply through out-of-game RMT, that spoils some of the fun of many role-players and collectors - and we are all role-players to some extent: we are trying to kill monsters, not move pixels - and we are all collectors to some extent, even if what we are collecting is boss-kills.
Finally, to return to the original quote from Tobold, above, I doubt there are many people who think that WoW is mainly a game of player skill (though of course there are some who do not possess even the small modicum of situational awareness needed to GTFOOTF). Of course, some players are more skilful than others, but it is character skill that distinguishes a level 1 mage from a level 80 mage, not player skill. Player skill only matters when the characters are notionally equally skilled and geared.