The only reason would be playing games online, old firmware gets locked out a couple weeks after an update releases.
But there’s also no real reason not to if you’re already running CFW. As long as Luma is reasonably up-to-date, a system update can’t break anything.
If you haven’t already, definitely give 4 a try. The gameplay isn’t as polished, but you can see where everything you loved about 5 came from. And the story/characters are arguably even better than 5.