Never!

actually i have seen a major change with an order that came in for a boeing 737 window to be etched and mounted, the pdf was in 3D and could be rotated on page

Which means the update was a success. They kept the user experience the same, while introducing new features and supporting multiple operating systems.

Or they just update a lot of small things all the time so that you don't notice in till you need it

its because adobe never has time to upgrade acrobat they spent it patching the 6 new 0 days they introduced with the last update