Mr_Whiffles: I've always loved the idea behind GOG Connect, but I don't think it is very useful.
Useful to whom?
First you need to understand why there is such a system in the first place. No, the meaning is not that people would routinely buy games from Steam (only), and the redeem a free additional GOG version here. That wouldn't make much sense to GOG as a business, now would it?
The system is in place so that from time to time GOG can lure maybe some more Steam-users to GOG, and also keep them coming back to GOG to check whether there are any additional games in GOG Connect, and maybe they accidentally also actually buy something from GOG while they visit here. That's why GOG Connect activations seem to happen during big GOG sales.
So if you thought the idea was that you buy nothing from GOG but only from Steam, and then come to GOG from time to time to reclaim free additional GOG versions on top of your existing Steam versions, you are thinking it all wrong. That is not the core meaning.
Themken: I have also thought for a while that connect should be buried. If it encourages people to buy on Steam instead of here, it is not a good thing.
That is why it is so sporadic and random, so that people can't count on that ("buy from Steam, and get an extra GOG copy later for free.").
For instance, back when The Witcher 3 appeared, some people even here in GOG suggested that it makes more sense to buy it from Steam because "surely" you can later get a free GOG copy too, just because you could do that same before with The Witcher 1-2.
Maybe that is why GOG hasn't enabled such ability yet (redeem a free TW3 on GOG is you have it on Steam)? Not sure if ever, maybe when TW3 turns 10 years old? (Or then I am completely out of the loop if GOG has allowed that already...)