Patches generally patch everything from a version to the version it patches. This means you don't need every patch to be up to date... This is either full file replacement, or delta patching where differences between versions need to be changed, then each successive patch is applied until it's up to date.

But patches and older versions should be somewhere, accessible. Quite often for most other software like FireFox or VirtualDub it will show you the newest version, and then there's a tab for 'older versions' where you can download all the previous versions if you want... Some versions of the software incorporate bugs or unwanted features... Or perhaps work better on different OSes...

I wish a number of games didn't require you to download the whole game over again when there's a patch...

GOG's stupid patching system would disagree.

From what I heard, this isn't the case for Divinity: Original Sin.

My experiences so far don't disagree... I have never had to run two different patches to get a version up to date... Course more often than not i have to re-download a 4 Gig game rather than get a 30Mb patch or something...

Not quite right. Original Sin started (when it went on sale for everyone) as v1.0.5something. The current patch is v1.0.74+ to 1.0.107

Now I got this 1.0.107 hotfix that didn't install on my machine, because:

"[i]This patch does not support your version of the game
please download a new installer[/i]"

According to another user (can't find the posting right now) I missed the first patch that was released after I got the game. I installed every other patch that followed.

And then there's this answer from JuriJ from GOG support, who told another user to download a new installer if he did not install all the previous patches -> http://www.gog.com/forum/divine_divinity_series/1107_hotfix_does_not_support_my_version_of_the_game/post15 So, yes, you can miss patches on GOG ;)