For me the main use of UnMHT is not to save MHT files, but to read them. On my work desktop, using Microsoft Outlook to read mail, you can use Firefox with UnMHT as a workaround for Outlook's horribly slow HTML renderer. Outlook shows 'click here to display this message in a web browser'; by installing UnMHT you can associate Firefox with
.mht files so this action opens the message in Firefox. Otherwise you're stuck with Internet Explorer or possibly Microsoft Edge.
(There is a trick you can use in your HTML messages to make sure Outlook always shows that prompt to display in a browser. So at work, when generating HTML to spam out, I always include that. Then messages which render too slowly in Outlook, because of large tables for example, can be viewed in Firefox with a right-click.)