Posting screenshots with tweaked out desktops in our answers is just unnecessary extra thinking/confusion for the target audience of a support site. Screenshots should feature a more-or-less stock install.
To be clear, I'm not proposing that we remove customized screenshots or ban people from posting them or anything like that. But that we should encourage stock screenshots and replacing screenshots should be something we encourage editors to do.
I agree with this - Everyone is of course absolutely free to customize and tweak their installation, but this is not what it's about. It's not a "brand" issue (but it's a nice side-effect, of course).
We don't know anything about the person asking for support - Neither their technical knowledge respectively skills nor their current setup. We should assume they're on stock and their skills are minimal. Even when this does not apply to the questioner, it might apply to people looking at the answer later on. There are quite a few people out there who do face problems when things aren't looking the way they're supposed to do (I guess everyone knows the stories about people saying "I deleted the internet!").
elementary OS is supposed to be an operating system even for the more-or-less computer illiterate - By providing "standardized" instructions, we make problem solving for all kind of users easy. Some are already "challenged" by the information / fix itself, let's not make it harder for them (by providing various looks and settings in screenshot instructions).
Users will customize their desktops. It's an option they have, so it will happen. It's quite difficult to police this when a site gets large.
I say, encourage users to post screenshots of stock setups and editors to replace screenshots if needed/possible, as you said... but remember that sometimes the theme ain't gone make no difference if the app just works the same way anyway.
