I used to. When I was first hired on, it was to help implement FreeBSD-based firewalls in the secondary schools and all admin sites. So I needed to learn as much about FreeBSD as I could, in the shortest amount of time. So I installed FreeBSD 4.something on a desktop, along with KDE 2.something, and forced myself to use that as my only workstation, for everything. Rest of the staff was Windows 98SE, some Windows XP, and some RedHat 7/8/9/thereabouts.
I kept that workstation running FreeBSD for several years, until we had standardised on Debian Linux for everything, at which point I needed to learn Debian inside and out, so I installed Debian on my workstation with KDE 3.something.
I no longer run a FreeBSD desktop at work, but do at home. And we still run FreeBSD on our firewalls, with FreeBSD on our storage servers as well. Everything else is Debian Linux or Ubuntu Linux.
I still believe the best way to learn an OS is to force yourself to use that OS as your primary workstation. No dual-boot option, no VM option, no emulators. Just that OS and the apps for that OS. Make it work. Once you are comfortable with the OS, then you can add in dual-boot, VMs, etc to make life easier.

If you have an "out", you won't learn.
