Unless you're planning on using the Apple Magic Trackpad with your laptop, it's hard to try and find reasons why this new peripheral can only use batteries. It's true this use of batteries is wasteful, and a USB-version would be much more appropriate. Until Apple comes up with this option, you can always hack your Magic Trackpad.
For now, Apple has neglected a wired version of this device. Markbog decided to convert his new trackpad so that it could run off USB power. It's a must for any desktop peripheral, although with USB ports few in between, maybe batteries make sense in some cases.
This is a very simply hack that requires a wooden dowel (cylindrical piece of wood that will fit in the trackpad's battery compartment). The dowel needs to have screws on each end. You'll also need some USB cord adn spare wire. First up, remove the batteries from your trackpad. Then, strip the USB cord down to the red and black wires. Attach them to the dowel and you're done!
USB power supplies 5V, which is more than the 3V 2 AA batteries provide, but it seems that the trackpad is able to regulate this power. There hasn't been an indication of it malfunctioning yet. While this trick seems to work well for the Magic Trackpad, we'd be careful about using it for other peripherals without adding some resistors or ways to modify the intensity of the USB current. You can end up bricking your device if the USB delivers too much power for it to handle.