Then again, most of what one would want from an API is available through regular web request.
Yes, you can certainly 'hack your way around' most things on GoG, but from a 'customer friendly' point of view there's plenty to gain from an API. First and most important, I'd like to mention that GreaseMonkey can cause issues on Windows 7, if you're not running as administrator (which you really shouldn't, after all, you wouldn't run as root on Linux for every day tasks either ;)). Now as for how an API could benefit, I'm just going to mention a very few points:
*Log-in with your GoG ID (perfect validation and not having to create yet another account)
*Get somebody's wishlist/shelf (if not disabled in privacy options) without synchronisation on your server
*Gift a game (no code/email fuzz)
*Send a GoG PM (again unless disabled)
*Save poor GoG some traffic (an API call should be far more lightweight than a full fledged page request)