Updating is not required unless you need a new feature or a specific bugfix from a new version.
But some of us just like exploring the new features.
Now, I make sure to update daily from the development sources.
You don't need to reinstall a plugin, but if a plugin gets updated you may want to update that -- bugfixes and new features for a plugin are separate from calibre.
The only time plugins needed to be updated
because calibre was updated, was for calibre v2.0 and that is because calibre moved to a new graphical toolkit (Qt4 --> Qt5).
There was a big announcement and everything, so users had some warning.