What's GOG's policy on hosting patches or mods that are often thought of as "essential"? I know I would enjoy having them available as a game goodie. Maybe it's just that it's tough getting the author(s) to give written consent or something like that?

If a patch or mod is required for game's stability, it may be included with the installer itself (see Gothic 3 and Might and Magic IX). If it's not required, GOG will likely do a mod spotlight and direct you to the proper source to get the mod from.

You get into a whole realm there, for instance - what is "essential", I can't play Oblivion without 50+ "essetial" mods. I don't think GOG can out resource into finding out what is "essential", testing, supporting, aquiring rights for etc. of mods for each game. You have excellent sites devoted to mods though, moddb, nexus etc. It is also up to the players choice what they add.

Another point being publishers themselves. If they see a market for patches on their software then you end up paying for a new "enhanced" version, like BG1+2.

Just put the effort in yourself to find and apply what patches/mods you want and you will appreciate it more.

Mod spotlights get lost in the forums, I wish they'd add the section directly in the game page.
Also, I didn't know that some titles had the mods already included inside the installer. Is that info written anywhere? Is there a list?

No idea, and I don't recall where I encountered said info. Gothic 3 uses (or used) the 1.74 community patch, though the latest one is 1.75. Can't recall about Might & Magic IX though, or if it was that one after all.