Gevlon wrote a post saying essentially that PLEX was cheating. PLEX is essentially a a game card for 1 month of game-time, that is purchased from CCP for real money, but can be sold in game for virtual currency. Gevlon was promptly jumped on by Eve loyalists in the comments. However, I think he is sort of right.
Though, "cheating" is not precisely the right word.
Rather, I would say that PLEX is against the spirit of Eve Online. Eve is a sandbox game, a virtual universe inside which one builds a virtual existence more or less from scratch. Allowing outside transactions to affect transactions inside the virtual universe seems to cut against the whole sandbox idea.
It kind of comes back to the notion of inconvenience. It is certainly convenient for, as an example, PvP pilots to buy PLEX to fund their games. But there is a price for that. Those pilots don't have to work, to participate in the game economy, for those funds. If PLEX did not exist, they would have to do something, produce something of value, before they could get to what they think is the fun part.
Maybe they would take up mining, maybe they would become pirates. Who knows? But not having to do that does distort the economy in some small fashion. It weakens the notion of Eve as a self-contained sandbox universe.
But maybe PLEX is a necessary evil. Maybe its existence spices up gameplay, and makes the economy more liquid, with more and larger transactions moving around. Maybe the alternative is those players quitting Eve instead of continuing to play. Maybe it means that CCP would make less money, and not be able to fund development as Eve deserves. Maybe third-party ISK sellers would fill the void, leading to the same result, but with a lot more unfortunate side-effects.
Ultimately, inconvenience is what makes a world a world, and not just a game. PLEX is convenient, but makes Eve less of a world, and more of a game.