Most sites that have Gears support don't force the user to install Gears to use the site. There is no pop-up telling the users that they should get Gears to run the site - Google docs just has a nice little link at the top that says "Offline". There is no impetus to click it, and I bet many users don't even realize this feature exists. When the plug-in becomes required to use a site, and the user is presented with a pop-up telling him so, I think that changes things quite a bit.