Well I'm far from being an authority on this but:
18elders said: "the tightness of the stance is used to check, trap or attack your opponents front leg and also protect your groin. the extended stance doesn't offer this."
You may be correct in that the extended stance doesn't offer this but I believe in the context of the form you are using the extended version of the stance to trip your opponent (your lead leg behind his) with a waist chop motion or punch.
shang wu said: "the stance I think we are talking about is used in waist chopping so in the right place it is not vulnerable to a sweep"
See? It's all about using the right tools for the job.
flem said: "the Pong Lai version seems as though it could easily be "over run" with a lunge or charge forward"
I was thinking the opposite. Since their momentum is moving forward what braces them from a pulling action?