It's nice having bonuses, but I hardly ever look at them. While I would like getting more extras on GOG releases, I wouldn't want GOG to reject games because the dev refuses to include the soundtrack on the package.
I agree with you --I've downloaded *all* of the bonus content for all the games I have purchased, but I think it has only been maybe a handful of times I have actually opened any of the extra content out of curiosity.
In some cases it was to hear a track or two from the soundtrack or a game which came with a "making of" video (which I will watch if available).
Having a DRM free game is important; about the only bonus material I would probably really need is a manual for one of the "Good Old Games" which never came with integrated help or tutorials.