ChrisSZ: Why not use the GOG Wallet? Then you only need to go through the payment process once to load some money into your wallet. Saves credit card fees, currency conversion fees, circumvents the annoying 2-step-authentication process for every single purchase and makes it much faster.
Well, the short ... and probably long ... answer, is that I don't believe in money sitting around waiting to be used, that is not in my bank (Credit Union), but some other site that cannot give an absolute guarantee. I don't even have funds in PayPal, unless it is due to a refund.
Like many years have shown me, don't trust what you don't have to ... just for the sake of more convenience. Many many people have gotten their fingers burned, and I'm not one of them. It must be remembered, that the GOG Wallet is not an account you can draw from, except to buy one of their games.
While I expect GOG to be around for a good long while yet, you just never know.
And what if I got run over crossing the street tomorrow. Is my family going to remember my GOG Wallet funds, and even if they did, and are savvy enough, can they get the cash back?
P.S. I also know from money that has ended up in my GOG Wallet, that I am more likely to buy something, because the urge to use my wallet funds competes with my decision making judgment. Sometimes it is not smart to make things too easy.