Earlier on, I talked about CCP considering allowing PLEX to be used to pay the way for fan events. Well, only being about, oh, nine and a half months LATE, it turns out CCP is already fine with players using PLEX for fan events. Specifically Fanfest 2012, where PLEX could be used for access passes of various style, plus fanfest events of many kinds, including a hoity toity dinner and a chance to get a behind-the-scenes tour of CCP itself.. What I should have said, is that PLEX is considered being allowed to pay for PLAYER created/run events, like Eve Vegas, or ones anywhere in the world that are quasi, or fully, CCP sanctioned.
On one hand, it's cool that CCP allows players to subsidize a fanfest event by working on the cost in-game. At the same time, it takes an in-game resource that was intended for subscriptions, and drives the price up because more uses for the PLEX increases the demand and increases inflation. increasing the PLEX prices to the point that players are no longer interested in, or no longer have the time to earn the isk to pay for the PLEX, means they will either pay for PLEX with real money, or stop playing Eve. People who heavily rely on PLEX for subscriptions, will opt for not playing anymore. Pretty much defeats the purpose of PLEX in the first place. PLEX for subscriptions should be a CCP priority. PLEX for extracurricular things should be a very distant second. If CCP stays greedy and allows PLEX for anything and everything, they'll eventually pay with subscription loss.
Accepting ISK, on the other hand, or partial PLEX and partial ISK for non-subscription things, means they still make extra money off the PLEX, they might be able to manage PLEX inflation, keep subscriptions and they would provide a big ISK sink for Eve.
EDIT: Also, just noticed, but PLEX are not $15 each, but $20 each. Not sure when the price went up, but it's been a while since I bought any with real cash. That's an extra 25% increase in the money CCP gets every time they spin a PLEX-for-something plan.