Presentation Rules (e.g. 7x7 in Undergrad Communications Courses): are they supported by evidence? I taught a Business Communications course that brashly asserted that good presentations follow the 7x7 rule (max 7 lines of text per slide, max 7 words per line). I also "learned" this in my undergraduate communications courses at a fairly decent state university (and in some MBA prerequisite work). 

I see docs online at Columbia (and, other, sources - this one linked to by MIT...)that hold forth in the same manner.

I did find this bit of research that may give some credence to the above doctrine, although it failed to reject the null hypothesis, that there is no difference in number of lines per slide, in favor of the idea that retention was better for 5 lines per slide versus 10. (But it's still something, some kind of evidence, right?)

So if the evidence is spare and specious, why are we reciting it like it's the gospel truth?

In industry (and to a lesser extent in graduate level education) we sometimes see vast amounts of information contained in single slides, and it is unclear what is the specific downside of this, particularly when well-warranted (i.e. the visible whole paints an important picture that would otherwise not be seen viewed at finer granularity, slide by slide). 

A lot of information is packed into those slides, but I print them off one or two to a page. Decks don't get much bigger than a dozen or so pages, and the information I'm particularly interested in may be packed into one or two slides. And the audience is fairly sophisticated, whereas the accepted introductory wisdom might be targeted to more average audiences.

So the question is, are these heuristics worth sticking to? Or are they artificial constructs designed to give criteria for ranking the ability of neophytes to follow instructions?

Or are they a little of both?
All axiomatic principles with respect to presentations should be taken with a heavy dose of salt, not just a pinch.

The important issue at hand is that you need to communicate your information to an audience effectively. Putting too much text on a slide makes audiences read the slide, and possibly tune out your elucidations and elaborations of the material on the slide. Similarly, putting too many graphs on the same slide causes the same problem—there's too much to focus on, so you get "lost" in the course of the talk, which again is entirely unhelpful.

Principles such as the "7 x 7" guideline are an attempt to balance between having too much and too little information on a slide. They're useful as rough guidelines, but need not be treated as strict rules to follow.