I still use MS-DOS on occasion. Mainly due to certain legacy products that need support and where the software tools were written in MS-DOS and only run on a machine booted up as such.
Back in the days when Windows was taking off, I was a very late hold-out, and continued to use only MS-DOS, other than just playing around with Windows 3.1 on occasion. When Windows 95 first came out, I finally made the switch, with my primary machine being Windows and my secondary machine being MS-DOS.
These days, it's Windows 7 Ultimate 64-bit, with Windows XP Mode for backwards compatibility for certain applications. In addition to a Linux machine and some Macs for doing iPhone/iPad development.
I don't know why anyone would still want to use Windows 3.1 as their primary OS. It would be so limiting, being any new applications wouldn't likely support it. I could really only see it being used if necessary for some backwards compatibility with old applications that there was no new version for, or if the cost to upgrade to a new version was prohibitive. Of course if you wanted a working vintage computer of that era, then it would make sense to utilize Windows 3.1 on it. But for a modern PC, I just don't see it.