Yeah, I probably wouldn't sign up just for this either. First thing that got me on GOG was Sid Meier's Alpha Centauri. My old cdrom looked like an ice rink at the end of the night and ebay had copies of the planetary pack going for $75 and up. So when GOG put it up for $3, I was all over it and now any time they offer freebies, I'm all over them. I got 3 free old fallouts that way.
Steam has also gotten more annoying over the years. Forced game updates before you can play reminds me of microsoft and consoles. I'm not completely sour on Steam (that's probably where I'll get Fallout 4), but where the price is the same, I'm definitely now biased towards GOG.