Yeah, the system was just recently started on and isn't finished. Glass is used to communicate what mods are needed, and will open a prompt for you to download these automatically before joining the server. Of course, if you don't have Glass installed on the client, your client 1) can't tell the server if you have the client or not, and 2) can't open the prompt to download them.
I'm working on a support file that can be put in to Client mods so that they can communicate what clients are installed without having Glass entirely, so that users can just download the client mods without glass. However, since that isn't done, it is how it is now.
I want to restate, this functionality is only found the the alpha, in-development version of glass. And it's entirely optional.
Users who don't understand the nature of an unstable in-development version shouldn't download it. Unfortunately, people run toward the shiny and "new" version without understanding that there's no guarantee that it'll be functional and complete. I even added a prompt that says in big letters "stuff will be broken, please read the development topic".