#1 reason to use an OS: because you want to.
#1 reason to not use an OS: because you don't want to.
Pluses and minuses aside, push comes to shove you use what you love (even if it's considered a POS hobby OS).
DR-DOS (at home)
FreeDOS (at home)
Linux (Redhat/CentOS/Fedora/Debian/Arch/Slackware/Ubuntu/etc) (professionally)
Windows 3.10/3.11 (home)
Windows 95 (home)
Windows 98 (professionally)
Windows ME (home)
Windows NT (professionally)
Windows 2000 (professionally)
Windows XP (professionally)
Windows 2003 (professionally)
Windows 2008 (professionally)
Windows Vista (home)
Windows 7 (professionally)
OS X (professionally)
A/400 (trained, never ended up using outside school)
And a few besides that that I've forgotten about.
And based on this experience I can say that without a doubt that these reason 'to use' and 'not to use' seem like they were written by a 15 year old whose experience involves setting up a web server on a old PC using FreeBSD at home and fooling around with it for a few months.
I meant blind Linux fanboys, who love Linux only, because it's Linux and ignore everything else.
Linux is for l4m3rZ who toy around with dumb stuff like HPC, 3D modelling, photo editing, virtualisation, UI development etc. FreeBSD at least is for professional tasks such as learning vi and configuring the system to make USB sticks work.
What the fuck does that even mean?It's easier to run Mac OS X than FreeBSD.