Most of the projects I think of and don't build are scrapped for want of $50 worth of ridiculous "zigbee" wireless hardware that can barely push 250kbits across the room. The internet of things is made of wifi.
Also, You'd be hard pressed to get a USB Wifi Stick for less than the cost of the whole Raspberry Pi board in my country. Plus as you add more devices Wifi can get very saturated. Installation and configuration of USB Wifi adapters under Linux is a bit of a challenge. Ethernet on the other hand is practically plug and play.
There are also plenty of ideas which can use wired networking. Ideas from home automation to set-top boxes to microservers are all going to benefit from a more stable and high-bandwidth wired connection.
For general tinkering I would rather pay the extra $10 or whatever and get more options in terms of the hardware.
It has a MIPS CPU, has built in 802.11b/g/n, 1 ethernet port, a USB port, and runs at low power off a USB jack (5V). It can be flashed with Linux but you have to navigate chinese menus to do so (not hard).