because you have to resync when switching. When you sync to a PC, its going to stay synced until you change it, and vice versa.
I play both my PC and 360, and every time i switch between them, i have to press the button on the console/wireless receiver, and the button on my controller.
If you only use it for the 360 or just for PC, then it would work perfectly.
The wireless receiver is only good if you already have an wireless 360 controller, if not, then you would be spending $20 more than an wired controller.