Idea #1: Ferry service for low-level players
This is a good candidate for intended purpose, as it's a way for lowbies to travel with someone higher-up on the chain, perhaps to zones that can be used to power-level more easily than others. In the time-honored tradition of appropriating a player exploit and modifying it for inclusion in the game proper, the ferry service could be used to level alts to the point where they'd make good raid partners for a guild. Whether or not this is actually the reasoning behind this mount feature remains to be seen.
Another benefit to this is the opening up of flight paths for lowbies that they either couldn't find or couldn't get to on their own. While one might question the actual practicality of this, some people are completists -- they play to see it all, even if they don't want to play it all.
Idea #2: Travel guide
This is the bigger, broader idea: to act as a travel guide for people who just want to see the world without having to fight their way through it. There are a few professional virtual travel agencies around, including Synthravels, who we previously covered with Second Life Insider. Although their website has dated information and we don't know if it's still actually in operation, this update seems custom-made for their business.
Imagine, if you will, that you're a journalist who wants to write about WoW, but you've never played before and aren't truly interested in getting involved. You just want to get in, get the info, and get out. Why not use a trial account to take the tour; the only obstacle of any kind would be the download time, which could be planned well in advance of any tour date.
WoW has some gorgeous vistas worthy of a visit, and a ton of content that the casual player may never get to see. When so much time and effort has gone into creating a world of this depth, it's a shame to not open it up to the rest of the non-game-playing audience. And, thinking far enough ahead, if/when WoW gets its updated graphics, it could be a whole new world for even experienced players to want to revisit.
Would Blizzard allow for this sort of outside-the-box usage of its new mount feature? We'd like to think so. There's nothing inherently illegal about it, and if it serves to interest more people in checking Azeroth out, all the better. Players, start laying the groundwork for your new business today!