Nintendo has just announced at Nintendo World 2011 why the new Nintendo 3DS includes a charging dock. The reason is that the device has a 3-5 hour battery life when playing 3DS games and 5-8 hours when running DS games. Keep in mind that the latest Nintendo DS, the DSi has a 9 to 14 hour life, and the original DS had a 10 to 14 hour battery life. It also takes 3 and a half hours to charge. These numbers are all from Nintendo and could be a bit worrying for the new console.
Though, as Joystiq pointed out, it does have to power more than one display and keep the wireless connection active, so there is a reason for the lower battery life.
The 3DS is currently scheduled for a March 2011 release date and for the latest news stick to techgeek.com.au.