I've been following Gevlon's comment thread, and I've realized that he and I are actually talking about two different aspects of PLEX.
PLEX versus Economic Simulation
From the outside, one of the main attractions of Eve Online is that it is this huge economic simulation with hundreds of thousands of actors. People harvest resources, refine resources, produce goods, trade, purchase and use those goods. Then you add on the player conflict overlaid on top of all that. It is a magnificent economic simulation and experiment.
It is amazing to realize that the ammo I purchase to shoot pirates was produced by another player, as is the ship I fly in and modules which the ship is outfitted in.
The thing about this economic simulation is that you can trace the flow of wealth, see how all the interlocking transactions combine into one harmonious whole. And each transaction makes sense within the universe.
But then, inside this beautiful simulation, you have some extremely large transactions that simply do not make sense within the context of the universe. Half a billion ISK transferred from one player to another, for no discernible rhyme or reason. That transaction weakens the economic simulation, warps it slightly, has trickle-down effects, and makes the whole thing less real than it could be.
PLEX as a Means of Skipping Content
Some people are producers. They enjoys earning ISK, either by harvesting resources or trading or producing goods. They do not enjoy being attacked by other players. But they deal with that inconvenience, and adjust their gameplay to defend or mitigate against that possibility.
PLEX buyers are consumers. They enjoy expending ISK, often on attacking other players. They do not enjoy earning ISK.
However, unlike the producers, the consumers don't have to deal with their inconveniences. They don't have to adjust their gameplay to compensate. They can spend real money to skip the part of the game they don't enjoy.
That is the part which is not fair. The situation is not symmetrical. One faction can skip the part of the game they do not enjoy, while the other faction cannot.
Consider the following hypothetical. Suppose CCP sold, for real money, an IMMORTAL module which rendered your ship immune to player attack (but disabled your weapons while installed). The module would have a lifespan of one month of real-time, after which it would self-destruct.
Would the Eve playerbase be okay with such a module? Or would using it be considered cheating?
After all, all it does is equalize the situation between producers and consumers. The consumer can buy PLEX to skip the parts of the game he doesn't enjoy. The producer can buy IMMORTAL modules to skip the parts of the game which she doesn't enjoy.
Yet I suspect that the Eve loyalists would howl if IMMORTAL modules were ever sold by CCP. But in many ways IMMORTAL modules are no different than PLEX in their effect on how an individual enjoys the game.