I've seen occasional problems and I would guess they relate to sites that are using plugins that stop working or who have hacked into the WordPress core. I've done both in the past, though was aware what I had changed so I was able to put my changes back. I'm sure there are some who had someone else work on their site and didn't realize a change had been made.
No reason you have to upgrade WordPress with each new version, though it is a good idea to see if any of the changes are specific security fixes. When that happens it's easy enough to grab the couple or three files that are security related and replace the old with the new.
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