This is going to be a welcome change. Especially since this will work exactly like the old IE fallbacks. I'll be keeping an eye on which organizations use these in their emails and which don't. :)
Email hasn't exactly kept up with web standards for a few reasons, but mostly because Microsoft Outlook decided to use Microsoft Word as the rendering engine for email instead of a web browser. For the longest time, emails were built using tables and, for the most part, still continue to be constructed using tables. Some HTML elements are fine but there's nothing that works really consistently well to make sure that your emails show up the same way in Outlook as they do in webmail clients or other desktop email clients. The solution up until now has been to include all this extra stuff in a section of the email and then trick Outlook into reading a simplified version of the email while still allowing other email clients to read the full version. It's basically a trick that lets you do some of the fancier stuff in email while making sure that the content is at least readable by people using Outlook, even if they don't see the fancy stuff.
This same situation happened on the web for Internet Explorer. Nearly every modern browser supported a set of standards that IE just wouldn't deal well with and so people did the same thing and made the websites work for all the other browsers and then used various tricks to get IE to play nice. Media Queries on the web were one such case. IE couldn't really deal with them and so people would make media queries for their sites and then create separate styles just for IE. It was a huge pain. I'm just curious to see how this extends now that we're back to a change like this in email. Outlook is now web-based and Microsoft is way better at keeping up with the rest of the industry but I can't help but think that there are still crowds of people using older versions of Outlook.