As an example, I had a BB 8800 for about 2 years or so. Upgraded from OS 4.1 to 4.2 for some reason. 4.1 wouldn't do something I needed/wanted to do. After that it worked just the way I wanted so there was no need to go to 4.5 when that came out. Nothing was broken, so there was no need to fix it
If you wanted HTML emails it would've been a damn good reason to upgrade. Blackberry's are so much better with that feature IMHO.