Some good comments from this thread:
You can't actually tell people they can directly buy XP increases. You have to setup something to obscure the issue and pretend it has a legitimate usage...
*cough* WoW recruit-a-friend *cough*
In WoW or WAR I am on the same turf as everybody else. My character isn't limited to my bank account, my status, my job, be it good or bad. This maintains the fragile illusion of these games, that you are in fact someone else. This shatters completely as soon as you bring reality (in this case money) into the game. Be it micro-payments or macro-payments, the alternate reality is broken and dead. Spock no longer just have a little beard, he also has purple hair and moonboots.
They are also going to fuck people's sense of achievement. I read in Predictably Irrational (or Freakanomics, I forget which) that as soon as money is put on the table, people consider it a financial transaction, and disregard any intrinsic motivators (which is why it's really bad to tell your date how much the dinner cost, unless they would not be offended if a stranger offered them that amount of money for any favors you were expecting). Formally putting a price on XP will tell gamers what their time was worth, and lots of them will be pissed off that the price was too low.