It definitely is a rule-of-thumb. You may not think it's a good one, but it still is one.
> it isn't a good guide, and it is never appropriate. Except maybe as...
Claims as strong as those are rarely true.
One might as well try to defend scales as being a valid form of music that should be performed in front of thousands of people. (And I mean literal, raw scales, not something that works a scale into a piece, for which there is no meaningful equivalent in this writing context.) Even if someone did that, it would be as some sort of artistic comment on listeners or the nature of art or something (see 4'33"), not because they have any reason to be performed in front of an audience.
By the way, since this may seem a bizarre amount of passion on the topic, what cheeses me off is that we TEACH this as the One True Essay Format. (See the beginning of the linked story.) I just can't believe the unnecessary harm we do with this. Even with my relatively low opinion of the ability of current formal schooling to educate, this still surprises me as a bizarre choice. So if I sound like I'd like everyone reading this to come away with the idea that this format ought to have a stake driven through it and you should never use it, it's just my feeble attempt to undo years of terrible schooling in a handful of paragraphs.