Shadowcat: I think they've only pulled that stunt once (so far) with this game, but it's true that if you bought and downloaded the game early on, you'll get screwed the first time you try to update it. I suspect it's fairly unlikely that you'll need to download the game a third time, but who knows. Downloading it a second time still hasn't been practical, so I haven't played the game since that happened.

Anyhow, you've laid the blame on the developer, but my impression (and no one has contradicted me on this in the past) is that it's GOG who are at fault for forcing you to download the full game a second time, not the developer. It's clear that GOG 'package' the updates themselves -- they have a GOG-themed GUI installer for them! -- so my assumption is that for whatever reason* they couldn't or wouldn't produce a regular patch in this format, and instead resorted to forcing the full download on us.

* I rather suspect it's due to an unwillingness to use cumulative patches when a really big update occurs, because then there would be multiple patches, and they like their "one single 'universal' update" approach. This would be the worst possible reason for forcing people to re-download massive files, so I hope I'm wrong.