Is it really too much to ask that you get a $25 controller to use on your $1,000 computer when playing games? Especially when the tools are free. Personally, I think that the kind of games that XNA is suitable for making mostly play much better with gamepad than with keyboard support.
If you want to add keyboard support, it's not that hard. Add a KeyboardGamePad class, and make it implement the same functions as regular GamePad, but read the keyboard instead. Return -1/1 for the X and Y thumb sticks when the user presses WASD or arrow keys (left and right), space == A, enter == start, esc == B, backspace == back, etc.
Once you have the class written (an hour?) it's a matter of minutes to copy it into any sample game you want to play.