"blizzard is far too ethical to resort to having to promote or sell things for in-game rewards. because gold-farmers would ruin the game for the players who do the right thing, etc"
Three words:
Spectral Tiger Mount
http://www.wowhead.com/?item=33225#comments
sure, it's not 'in-game' currency, but the TCG rewards and the rewards from events like blizzcon and 'recruit-a-friend' are still very profitable for blizzard, just like that money made by farming gold 'legitmately' or 'not-so-legitimately' and then selling it.
it's a morality lesson that beguiles people who aren't exposed to the obnoxious fashion in which blizzard casually, but regularly advertises RM t-shirts, mice, card games, mousepads, events, vs. the way goldfarmers advertise their services.
MMO's selling IGE for profit, either come down 'our standards are so tough because we have at least two of them' or they don't have the means or the numbers to 'skim' politely off the top of customers, and rather, do so more aggressively or restrict users who are not paying to play the game.
eve online at least has the temerity to offer RW/IG exchange of money as a profitable resource, web games like travian and others offer donation/upkeep bonuses, etc. which are honestly tame compared to direct item-sales of balance-breaking weapons or bonuses that unbalance the game for those not paying their way ahead.
the problem is that the IGE is based upon several hundred people also trying to make money exploiting the resources of the game's economic system, for personal acquisition, or systematic gains, i.e. to buy more items to gain better gear to gain more items, etc.
given the number of people and players who already game the economic system already, introducing RM bonuses for levelling i.e. recruit-a-friend, introducing RM rewards, plus, in addition, they are already seeking to undermine the economic system in several ways to try to curb various inadequacies of the principles of any economic system, i.e. where does one spend money on consuming goods, if one has no need for goods beyond the means to trade money.
giant bags. exclusive jewellery. motorcycles. and mounts. RM/IGM consumerism.
i could be wrong, it might not be blizzard falling into a slippery slope of inducing people to buy goods, services, in-game-rewards, etc. but that's the point of small changes, you typically don't notice it until it's too late to change or interfere.