From a balancing standpoint, the purchased horse does nothing except allow some players to have only one button for a mount instead of 2. Why 2? Because, pre-"the next expansion", everyone cannot use their fastest flying mount in the old world. So, they have to have a second button for their "fastest ground mount." They are planning on doing away with this issue in the next expansion. (Both the use of flying mounts in the old world, as well as allowing someone to keep one mount for their entire life if they so choose... and have it increase speed.) With this said, I'll never own this mount since it requires real world dollars for only a few lines of code.
As for the MMO vs "other games" argument, MMO cost is easily compared to "going out with my mate" for me. We both enjoy the game, so we both play together (duo'ing). 2 subscriptions of $15 a month (if bought per month) = $30. That is the cost of one night of movie tickets and a drink if you go to the night showing. Which has more entertainment value for us... the current MMO we are playing. (We cycle through them.) Further, with the cycling, there is the added ~$100 cost the initial purchase of 2 copies of the game - $30 for the first "free" month = ~$70. $70 is 2 dinners at a non-"fast food" restaurant, and we agree to this before the purchase is made. In short, the MMO paid monthly subscription model works. (Real world example: Financing anything via a loan of X years, where you can calculate your budget and know how much you can spend.)
I, however, disagree with the "free to play, but pay for stuff" model. Why? Company X releases a new "uber X" that beats the last "uber X" they released all at a low low cost of Y. Therefore, you must pay Y in order to still be "the best of the best at your class/role"... or your guild will see you as 2nd rate and replace you with someone that can spend Y... NOW. (Real world example: Impulse buy)
As some others have stated, the problem is the difference in the Impulse buy and the Finance buy... and some people's lack of ability to budget their money.