Sure, but are they sending it for a concious choice to prefer HTML, or because it's Gmail's default?

Does it matter? The default is HTML, the majority of mail is HTML (or atleast multipart). So why does it matter if GMail defaults to what the majority is doing? Everyone (99.9999% of people) can receive and send HTML, so just do that, it's the informal standard for mail now.

I think it does matter. For my rationale consult TLA.

It's great that you think that it matters but I don't think that what you think that it matters matters in practise, really, the world is HTML email now.

The people you are interacting are not sending HTML email on purpose, but simply by accident because of details. I would hazard to guess that if the default was changed that they would not noticed.

Given all the communications that you receive, how often have typographic 'flourishes' been added in a useful way that would need mark up more advanced that ASCII/Unicode?

Given the following (from the article):

* HTML as a vector for phishing

* Privacy invasion and tracking

* Mail client vulnerabilities

* HTML emails are less accessible

What exactly does adding mark up give you on a day-to-day basis over a text/plain Content-Type?

Do they use formatting?

For myself and the people I work with, most definitely. Bold and italics in particular to highlight/accent certain parts of the text.