If you pay people minimum wage to do something they love and volunteer to do, they start to hate it and feel insulted. If XPs can be had for money, instead of seeing "You gain [amount] XP", you mentally translate to "You gain two cents".
tl;dr: Putting a price on priceless things makes them worthless.
Granting XP seems too much like pay to "win" and almost an invitation to spammers.
Also makes natural XPs worthless.
If you pay people minimum wage to do something they love and volunteer to do, they start to hate it and feel insulted. If XPs can be had for money, instead of seeing "You gain [amount] XP", you mentally translate to "You gain two cents".
tl;dr: Putting a price on priceless things makes them worthless.
Can't say I disagree. I could see some kind of XP economy developing which would probably take some of the fun out of the gamification of the site.