StingingVelvet: One of the main reasons I love GOG is the simple downloading and backing up of exe files. It's like MP3s but for videogames, which is how I feel all distribution should be. If movies and TV shows were sold in a similar manner I would buy thousands of them, so it's a shame they're not.
If GOG went to a client they would instantly lose a lot of support from people like me.
Having a "client" doesn't automatically means automatically having it required to download/install/play games. It could be an optional client; simply be a "enhanced" version of the GOG downloader.
Personally I would be 200% against a Steam-like mandatory client , but I would love having more features in the GOG downloader like for example being able to browse/search your collection directly from the downloader or at least having an option to check if the games you already downloaded were updated.