Again, it's very rare that a game's offline installer isn't the latest version, so in the vast majority of cases if you just want to back up, you ignore the patch files. I mean, plenty of GOG users back up their installers and they don't have this problem.

That's the problem - it's very rare. If it was like this for most /all games, I would consider just keeping all the patches (and wasting disk space for some of them). When it's rare, you either need to keep track of whether there's such patch for each game or just ignore patches and don't have latest version of game files on your disk.

People who back up GOG installers ALWAYS have this problem. They are either unaware of it or having incomplete backup by skipping necessary patches or wasting lot of disk space unnecessarily. I doubt anyone with large game library go through each game periodically to manually verify and download the necessary patches only.

Again, this is not something requiring insane resources, just a bit of common sense in file naming and / or rebuilding installers of games (done for most games anyway - why not all?).

When you say, "Incomplete", the implication I generally have is if the final version and the last patch result in an identical result, is there a difference?

Yeah, they generally just ignore the patches and only redownload the actual installers when they change, often with some automation for the process in case of those with large libraries.
And there's a question about the word "necessary" there. Like the "latest" version of the (now-old) Witcher 3 included Chinese translation, but it was a separate patch, the main installer being left as it was before that. And that was a good thing imho, seeing as that patch was quite large, so you only got it if you specifically wanted to play in Chinese. And now there are six separate language patches for the classic version, each multiple GB in size, so you only get one if you want to play in that language.
But a better example is that Hitman: Blood Money patch you mentioned that, based on what I saw on a quick search, may fix something (maybe with saves?) later but causes the game to crash after the first mission if installed when you start a new game, so you really DON'T want it incorporated in the main installer.
So, yes, GOG should add a little more information, file date would help, along with ensuring that changelogs are always available properly, but while I've been bashing GOG left and right for over a decade now for a lot of reasons, this one really is far lesser than you make it seem, and the "solution" in the title to abandon patches altogether is really the wrong approach.
Anyway, when in doubt, GOGDB helps, in the changelog tab, see when the main installer changed and when the latest patches were added. Also see the updates thread, and maybe the offline installer version discrepancies one and the table for it.

If you download installer and patches (or even only the last patch - but for some games, good luck figuring out which one is the last), then you choose this approach: because, for most games, the latest installer already have the latest patch applied.

Few good points there. Firstly, yeah, I already use gogrepoc.py script for automation. I also chose the installers-only path with just few manually downloaded patches that I'm aware of are needed to have the game up to date.
Regarding necessity - that's what I mentioned. A Chinese language pack for W3 shouldn't be called a patch, but language pack. Would make things much easier and logical.
And that HBM patch - just wow, you would think there should be screaming warning regarding that info. Maybe even file should be called optional patch and in game library on GOG there should be a warning regarding what you said. I mean GOG is mentioned to be delivering games that works out of the box, not another installers that require hours on google to make them work. Yeah, I know this is also the situation with modern games, but this is apparently a known issue that possibly won't be fixed - why no warning?
Finally, GOGDB is a good hint, thanks!