Why should you purchase a Battle Chest? You're already playing WoW! Well, imagine if more of your friends played WoW, too. This is a great way to get started, and do remember that, of course, given the recent addition of Wrath of the Lich King to the Battle Chest, and the change meaning that all players can play all races, there's a whole lot of WoW here for your money.
This is also a great way to get your hands on a mount, the Obsidian Nightwing to be precise, and as we investigated here at WoW Insider a while back, Recruit-A-Friend a pretty reasonable way to buy a mount. You also get some sweet leveling bonuses, grantable levels, 300%XP, that sort of thing, and you get the chance at a Scroll of Resurrection, when those next come around.
Update: The Battle Chest is now also on sale in the US for $10.