Because AppStore told me that I can upgrade.
I always reinstall and rsync my home directory back and reinstall applications on osx. I never have any of the problems people seem to have here. Just as an anecdotal point.
Upgrades "work" but honestly I don't trust upgrades on any operating system. I'm working on finishing up ansible plays to get things sorted out entirely.
Upgrade if you want, but accept you probably have skeletons in your closet.
As a note, here is how I do the entire thing, I rsync my home dir on each home wifi connect (scripted for years with ControlPlane), and midnight as well. I also rsync before upgrading, then I upgrade the os, rsync again, reinstall the os and rsync home dir back and reinstall vmware.
Sounds hard right? Takes about overnight really most for the rsync back and that just happens while I sleep. I do the same thing for linux/freebsd also. As an example, freebsd 10 upgrade had issues with bhyve not working after upgrading. For linux as an example, things can be markedly worse for things like ubuntu upgrades. Which is/isn't my experience as a (thankfully now ex) devops admin. Nothing is perfect, especially upgrades. That said I have tested the tires out on upgrades of OS X and not really seen issues that others do.
I understand no time, but setting this up, which granted for me is trivial as vmware is about it that I reinstall, the rest is in ~/Applications, means its trivial to not encounter dragons needlessly. I understand you might have the adobe creative suite or whatever installed and reinstalling might be worse than crossing into mordor, but this wisdom is passed on not just by me but from those that came before me. The rsync bit could be improved but I'm lazy and it works well enough.
And as for mactex it seems to work fine for what I use, though it installs to /usr/local/texlive when I tested it a month ago.