Straight spine alignment is pretty much ubiquitous in most "upright" martial arts. I prefer a more accurate terminology for this alignment: Hips locked, hips neutral, hips unlocked. So why lock the hips forward in WC? because it connects the upper and lower body...this is basic. Hips are unlocked typically while grappling, sending more of a concave energy characteristic from a lowered posture, as opposed to the convex hip locked in WC. Yes hips locked can cause the student to lean back a little to offset the hip tuck and this is not favorable for receiving hard force directed at the upper torso.
The problem is that many individuals have a naturally poor spine alignment along with bad posture. Their "natural straight" is often hips unlocked- butt sticking out causing a upper and lower body disconnection. The quick universal WC fix is the hips locked.
However there is another way to look at this. Depending on how you play your WC, especially during an active footwork round of chi sao you may strategically lock and unlock the hips...as needed. Typically in many WC when a student is caught unlocking the hips he or she is struck with a rolled up newspaper and told "NOOOO!" or a red X appears by your butt with a loud buzzer sound. By locking and unlocking the hips you create and modify your chi sao "work space", especially when dealing with chung chi or for drawing the opponent in.
Of course the golden middle is spine straight and body relaxed. Just food for thought from another prospective.
Buy the best and cry once!