Here's my take on spine tilt.
I do think that a good swing needs SOME spine tilt. This can be established at address by good posture. It helps in bringing the clubhead from the inside and keeping the right side down. Many amateurs, especially high handicappers do not have enough tilt.
However, I think you CAN have TOO MUCH tilt. Redgoat has talked about it here. Here are some disadvantages that I see with excessive tilt:
a) bad for the back
b) clubhead coming too much from inside with the bottom of the arc tending to fall behind the ball.
c) clubface coming in more open.
I will talk about these in a minute, but let's first compare the pics of Weir and Els that Redgoat put up. I've added some lines. For some reason the image got a bit distorted, but it is still useful in illustrating my points.
http://www.megspace.com/sports/jono007/weirvsels.html
The blue circle represents the head of the right humerus (i.e. right shoulder). The blue line is the distance between the right shoulder and the base of the neck (i.e. right shoulder lever). Likewise, the red circle represents the left shoulder and the red line represents the left shoulder lever.
The yellow circle is supposed to represent the pelvic brim ... you can think of it as the belt around the player's waist. The reason that I put this in is to get a more accurate idea of the player's spine angle. Many people draw a line from the base of the neck down to the belt buckle to represent the spine angle. This is not accurate (especially if the player has a large belly). The spine is at the BACK of the torso. It runs from the base of the neck down to the tail bone. That is why I like to look at the spine tilt angle from the behind view (ie. standing behind the player). It is easier to estimate the position of the tail bone.
The green line represents the spine tilt angle. This is a crude estimate.
You can clearly see that Weir's tilt angle is greater than Els'. However, if you compare the shoulder levers (red and blue lines), you can see that Els gets his right shoulder down just as well as Weir. This is because the shoulders can have some independent movement relative to the torso. The human torso is not a rigid cylinder. You can move the shoulders up or down by shrugging them without moving the torso.
Try this. Stand in front of a mirror with your hands by your sides. Left hand on the left thigh, right hand on the right thigh. Keep the spine upright. Now, WITHOUT tilting the spine, slide the left hand UP the left thigh, and slide the right hand DOWN the right thigh, by SHRUGGING the shoulders. Shrug the left shoulder UP and shrug (or deshrug, if you like) the right shoulder DOWN. Notice how much you can get the right shoulder down without having to tilt your spine to the right.
In a good swing, I believe that the right shoulder moves down because of: a) the spine tilt to the right AND b) deshrugging move of the right shoulder lever. Trying to achieve low right shoulder just by spine tilt alone, IMO, will cause too much spine tilt. Not only will this be bad for the back (increased torque and pressure on the spine), but you upset body's equilibrium and balance. As mentioned before, the clubhead will tend to come into the ball too much from the inside and the clubface will tend to come in too much open. Both require compensations from the hands and arms.
Re: open clubface with excessive spine tilt
Try this. Stand in front of a mirror whilst gripping a club. Stand upright and hold the club out in front of you at waist height (clubhead way off the ground at waist height). The leading edge of the clubface should be vertical (ie. 12 o'clock), with the club face square to the target. Now, TILT your spine to the right. More and more. See what happens to the clubface. The leading edge starts to tilt to the right along with the spine, and the clubface becomes OPEN. After you have introduced what you feel to be a lot of spine tilt (say 30 to 40 degrees), now LEAN over towards the ball to place the clubhead on the ground. Can you see how open the clubface is? As you introduce more and more spine tilt into the swing, you have to compensate more with the hands and arms to square the clubface. This will introduce inconsistencies.
Jono.