It doesn't have to be perfect it just has to work that is all. That doesn't mean that it has to be glitch free, quirk free, with the same features as the other two interfaces. All three interfaces have their quirks, their glitches and they all have various features which don't translate to the other interfaces.
In the end of the day, the Eyetoy sold what? 10M units on some of the most quirky/glitchy gameplay ever. Kinect has fewer glitches and performs even better than that and it appears to be getting significantly more support from both the manufacturer and 3rd parties.
It doesn't really matter how crappy the interface is. All the interfaces are crappy from at least one perspective and that encompasses mouse and keyboard, touch screens and dual analogue sticks along with Wiimote, Move and Kinect. They just have to be somewhere between good enough and good to allow for people to be entertained in using them. In the end the best possible outcome is that people get entertained in using the interface, the games will be doing that.
The ironic thing is that for some people the glitches would probably add to the enjoyment of the console. 








