I think you are missing some very major points:
1/. As in our case, there are companies that send information in html formatted email - at the SPECIFIC REQUEST of clients. Therefore blocking these at server level is doing something YOUR CUSTOMERS DO NOT WANT.
2/. Spammers have only one thing in mind, to get the highest action rate per email sent. Therefore if you are saying the majority of spam emails use html, then why are they doing it? - Obviously html email is more effective at getting a desired action than non html email. Therefore this proves the point that html email is more effective in certain situations than non html email (whether for spamming or legitimate purposes). If people like yourself just block html emails, then this would reduce the effectivness of the spam, and the spammer is not stupid, so as soon as non html formatted email becomes more effective than html formatted email, they are just going to switch over to non html format. Actually if I check my spam emails which I get a huge amount of, I would say 90%+ is plain text to get round spam filters.
since the majority of people who send legitimate e-mails don't have the skills to even MAKE an HTML one!
3/. You are not seperating the "general public" from "companies" here. Agreed the general public don't know how to make html, but the majority of decent web savy companies of course do - which is why you will find the majority of newletters type emails from these companies are done in html format (again they are not doing this out of fun as they take much longer to make - they are doing it because they know it is more effective).
And you think sending html to someone whose email reader doesn't know about html won't be a barrier to them? Or do you prefer just to pretend that those people don't still exist?
4/. If you know what you are doing it is relatively easy to send a html email, that will render as a normal text email would in a non html email reader.
5/. One of your points is that you should just provide a link to a webpage if you want html rendering. This ignores the fact that it is often the case that emals are often sent to companies which have a block ban on external links to any site not on their safe list. This makes any link useless, therefore there is a necessity to get that information in the email. Even if the customer can view the weblink, you are still putting an extra step in the process, which in any marketing is a number 1 rules to avoid.