I could most likely pull the presser foot from my 9W and use it on the W&W 8...and will consider doing that before I actually use the glass foot! (Thanks for the idea! :) )
I do think that the idea behind the glass insert is for visability...it's the only thing I can imagine. The metal where the glass inserts into the foot is where the feed dogs are, so it's not as if the glass has any other purpose that I can see...I'll have to get ahold of Miller and see if he knows for sure!