paladin181: It's not unnecessary to many players though. It easily facilitates on-line multiplayer, it allows sharing of statuses and social features such as achievements, screenshots and reviews and comments easily. While these things may not appeal to you, they do appeal to a LARGE contingent of gamers, and in fact are expected by many younger gamers today.
If you need these features, more power to you, I won't complain if you find joy in them, etc. But since I barely ever use the "Facebook" side of steam's community, don't need online multiplayer all that much (and if I do I'm willing to install a client so I'm flexible there), and don't care much for achievements, why am I then still forced to have the client on hand?
I'm not super into GOG Galaxy, but the biggest reason I like it is that either I truly want it or I don't. If I want achievements, online multiplayer, time tracking, leaderboards, all that jazz, it's here. If I don't feel like using a client, it's not here. For some reason, this is a luxury that only Galaxy provides, and not a basic feature.
I expect my things to just work. Achievements is yes, it's a nice bonus feature, but not one I'm willing to kill for.