what happened to plain email?

I've noticed lots of sites that I sign up with offer me the chance to receive email notification in either plain text or HTML format. I generally pick plain text. I figure it will load quicker, and they have less of a chance to throw a bunch of glitzy marketing crap into it. However, I was busy cleaning up my email archives the other day, and one thing that I noticed is that nearly every message had some sort of attachment. In most cases, when I receive an HTML-formatted message, Mozilla Thunderbird automatically assumes that it is junk or a scam, and blocks any inline images for me. I like how this works, as it means that I'm not loading anything unnecessary or potentially harmful, especially when I probably asked for a plain text version of the mail in the first place (and the sender ignored that request).

So what is going on here? I understand why businesses would opt to send a high-fashion HTML email full of flashy pictures and an eye-catching layout, but how may people actually load the images and look at them? And if they choose not to load them, what is left to see? Most HTML emails that I receive are entirely blank before I load the images. I've gotten to the point where I pretty much assume that anything with inline images is going to be some sort of spam, and I blow it away in many cases without even reading it.

So what can online businesses do about this marketing dilemma? I'm sure I'm not the only person out there who has this attitude of skepticism towards HTML-formatted emails. Why can't they just compose a nice succinct plain text email? I would think that the overall desensitization that we are seeing from obnoxious advertisements on the web would lead to the same approach we see companies like Google taking with their Adwords program. Namely, that text-based ads are used more frequently because they get better click-through. So how long before email marketeers start to realize this? My guess is that they're too busy trying to figure out how to squeeze a flash multimedia file into a email that they've forgotten the advantages of a good, solid message delivered in plain text.