The Silicone ADHESIVE that comes with the Polyglass is not just a simple silicone 'sealant' or gasket sealer. Using typical windshield Polyurethane or Butyl will not adhere well to this particular windshield's coating. This particular adhesive (made by Permatex) will NOT work well for other plastic or glass windshields either. Just the PolyGlass due to it's special coating.
Cobra windshield frames are not as sturdy without rigid glass supporting them. By gluing the PolyGlass into the frame, makes it into a ‘one piece unit’ that’s much stronger and more sturdy as well as preventing the pull of a soft top from pulling the frame away from the windshield. The recommended adhesive is made to adhere to the special super slippery silicone windshield coating used on this product, where others will not.
Even if you have a center rod support, it would be best to use adhesive to seal the PolyGlass in place, resulting in an overall more rigid windshield & frame assembly reducing deflection or buffeting even if not using a soft top. Just someone heavily leaning on the windshield could bend the frame where otherwise it would break the glass. Making it all 'one piece' makes the entire frame stronger.
While you 'can' omit the adhesive, you benefit from a better overall result using it. These tips and hints are shown on the webpage as well as others...
http://performanceunlimited.com/cobr...indshield.html