I agree, if we're only talking about straight legs versus bent knees. Now, if we start to bring into the conversation the *alignment* of those knees (i.e. whether they face into, or 45-degrees [or some angle thereof] away from the shot), that's an entirely different conversation altogether. In that respect, I do believe there's a big difference between
"your foot goes 'exactly' here, and both your foot and knee face 'exactly' this way," and
"just put your foot on the shot line, turn it 45-degrees or thereabouts, and that's sufficient." Big difference between the two!
Whatever you think you gain keeping your back leg straight is absolutely lost if you experience even the slightest pain or discomfort. Now you don't want to get too low by spreading your feet really wide and bending your knees like you're sitting down.... because you'll lose sight of the table.
IMHO, comfortably relaxed knees will actually add stability if you suffer from back pain of lack of flexibility.
I agree. Anything that is uncomfortable over the long haul is not good. (Save for at the outset, as when someone who'd never tried a snooker stance before and feels the typical pull in the calf muscle that goes away with practice/adoption of the stance.) Any body position that, over the course of time, continually "tells you" that this hurts no matter how often you do it (i.e. way past the muscle's initial "I've never done this before, so I'm not used to it" feeling), is not good.
I think of boxers, weight lifters and any activity that requires balance to be of top importance. Don't see many straight legged boxers.
I DISAGREE with this. This is the wrong analogy to use, on all counts. Boxers use bent knees for
mobility (i.e. the ability to duck, spring, bounce, etc.), not for "solid"/immovability reasons. Weight lifters use bent knees for
injury avoidance reasons (i.e. not placing the knees in a position where too much weight "pops that locked knee backwards or sideways"), not for "solid"/immovability reasons. These are not the reasons why a pool player should or should not use bent knees. The idea of a pool stance is to be comfortable, yet immovable / non-flimsy -- that latter part being key.
This Del Hill video describes it quite nicely:
http://youtube.com/watch?v=gSK4w_9S_x0
This actually goes back to the first point above, about alignment of the joints such that there's no sideways motion to introduce "yaw" in the cue delivery.
Hope this helps,
-Sean