Something just hit me as I am thinking about this walking around at work. The upper window felt channels on my truck don't have a metal channel around them, but the inside window frame pushes against them to help support them. But to tell the truth I ran my truck for a while without those mouldings and the windows went up and down fine without moving the felt window channels as they do have metal inside them.
This is the channel that I am talking about.
Automotive Window Channels - Glass Weatherstrip Scrapers - Car Weatherstripping Manufacturer UK
And how about a metal channel too!