physiology/mechanics theory aside, in the 10 years I've been riding with high angles on race boards, there's one thing I have noticed.
The closer your knees are, the faster you go and the harder you'll turn.
It's not that black and white though.
The thing to keep in mind is you and your gear. Naturally, there is a sweet spot for everyone's setup on a given day, and if your gear is tuned too extreme or too conservatively for your body, conditions, and riding style, then obviously your natural tendency will be to fight your settings to achieve optimal feel with your board. So you could find yourself pushing in or out with your knees depending on how you are tuned vs. how fast you think you should be going.
If you have strong legs and a need for speed, you might find yourself squeezing your knees on conservative settings to get every ounce of performance out of your board. What you may need to do is ride at higher angles with a rear foot cant on a narrower board, and you won't find yourself squeezing your knees all day. Conversely, if you ride your angles too high and you want a slower ride with more leasurely, thoughtful turns (or if you're in powder and you need to ride your rear foot a bit more), you may find yourself trying to pull your rear knee out for more control and a more stable stance.
Tight knees are good on groomers and ice and you'll ride like greased lightning, but wide knees are better for powder, and freestyle (and beginners) because as Crave points out, the mechanics favor control.
