Just an observation: Reader seems to have what I would regard as an "excessive" number of updates. No other software I own, other than perhaps Windows itself, has as many updates. I would think that at some point, they would have it "fixed" and patched, and can save feature enhancements for the next major rev. To my knowledge there is not a publicly available document that explains what the changes are in each minor rev.
I too had the same perception as yours until I came across the support pages for each patch/installer released by Adobe. Every release of Adobe Reader is accompanied with a corresponding support page and release notes. These outline the basic features/bugs which are fixed in Reader for that release.
It seems that every patch that they have released does actually fix certain security issue or some major bug, hence the importance of releasing the same.
For the latest Adobe Reader 9.3.4, you can find the relevant pages at:
Support pages: http://www.adobe.com/support/downloads/detail.jsp?ftpID=4761
This page outlines that the patch was a security update, details of which are enlisted at http://www.adobe.com/support/security/bulletins/apsb10-17.html
Any other information you require is also provided on the same support page.
Hope this changes your perception..