I bought Under a Killing Moon and Pandora's Directive on GOG some time ago, even though I have both games, somewhere... I think.
In any case, my motivation for paying for them again, was not particularly moral, but I feel that the effort GOG put into making them a) playable on a modern system B) not having to juggle 4-8 CDs was worth $5 or however much I paid for them.
I could probably have gotten the GOG installers on pirate bay for free, but with GOG and to a lesser extend Steam I feel we have reached a stage where paying for them is more convenient (everything considered) than pirating them, which is really where we should be in terms of piracy vs non-piracy.
Like Gabe Newell said, as far as he is concerned, piracy is just another competitor, and if they provide a better service than him, he's doing something wrong. (I'm quote from (a bad) memory, so I might have messed that up, but in my little fantasy world, that's what I believe he said.)