You are assuming that people save the installer for everything. And you can also archive Steam, so you just need Steam to install from the archive.
Depends on the person, but I think I have proved that it isn't as cut and dry as most of us would say.

Ah, I see. Not being a customer of Steam I wasn't aware of this. Thanks for the clarification.

Most people never bother because Steam games are updated rather regularly, rendering the archive nigh useless. :P

Archiving is pretty buggy, so sometimes it's not even worth it to create the archives.
Example: Team Fortress 2 archives may require you to verify the game cache after installing, or even uninstalling and allowing a full re-download.

The easiest way is just to copy the steamapps folder. Instant backup!

Actually, that's not how it works.
The GCF files you download are sort of like a zip archive. Steam verifies that the archive is the same as the current one. If it isn't, it compares the updated files to the ones on your archive and only downloads the files that are needed.
So for instance, Team Fortress 2, which has been updated -a lot- in the past two years... you don't need to redownload everything, just the new maps, new voice lines, executables, models, and animations. Most of the textures, music and maps remain unchanged. Hell, in some cases, the only thing you download is an updated executable.

This. I have my Steam library on 3 different computers, only installed/downloaded once.

Most of the responses in that thread are positive or at least curious. The haters and doubters are largely just kidding around. With Reddit it's sometimes a little hard to be sure.
That said, you have defiled the hive, get out.

I came, I saw, I posted.

Yeah I have to say that was the thing that impressed me the most about steam when I did a format & instal for Win 7, copied the appdata folder, reinstalled steam and was able to play again within 5 minutes

Hey the hive was asking for it! Look at the way it dresses!

nothing in the world is cut and dry
i know that there are advantages to steam that sometimes cross over into GOG's catalogue, but there are only four or five cases of that and it's still up for debate whether they even outweigh GOG's advantages
(GOG gets an almost unfair win in a lot of ways because of how old 90% of its games are--multiplayer is inconsequential and files are small and no more new patches are realistically going to be released so automatic patching is a nonissue)
because it's so overwhelmingly in GOG's favor, it's safe to generalize--anything else is nitpicking
and let's recall: i don't really think they should be compared at all.

Agree on the comparison, but people do it.
And I wouldn't say it is as overwhelmingly in GoG's favor as you claim.
GoG has extras. Honestly, most people don't care. I know I generally only grab manuals and soundtracks (if I like the soundtrack). Nice touch, but not a dealbreaker.
And, as you mention, most of the games are older. But that actually gives Steam an advantage (assuming the game works). The Steam Overlay. XFire doesn't support something like 75% of the games here (I pulled that out of my butt, but it is probably right :p). I think every Steam game has overlay support (well, except for R6:Vegas 1). So if you don't like to alt-tab out, you can still easily chat with friends and check for solutions/tips for the more fiendish puzzles, right form ingame with Steam.
Like I said, it is going to depend on the individual. So generalization just isn't really an option.