It would be cool if you could activate or register games that you already own hardcopies of on GOG.
I don't see that happening due to the DRM-free nature of GOG games. With Steam (which is probably where you're getting the feature request from), they can yank a game from you at anytime and they know you will no longer have it.
If a game is pulled from your GOG shelf (I don't see why this would happen), you still have the DRM-free downloaded copy to play when you wish.
The advantage to the publishers of having users register their game keys with Steam is that they are then locked to that account and other future owners of the hard copy cannot use them to play online. If the first owner didn't register the code, the game could be sold and without any issue for the buyer. If the key stays unregistered, this process could continue for quite a while.
In short, I don't see GOG doing this as their product is fundamentally different from original retail products. Just my 2 cents.