Is there any documentation about the features that require sessionState to be on in EpiServer CMS site? I know that Self-Optimizing block need's sessionState to work correctly and some features of visitor groups (don't know which ones) need also a sessionState.
We usually disable it per controller and haven't hit something major. Then again we also try to stay away from as many addons as possible.
I added this feature request a while back so not 100% on what I was thinking. Probably so that it would be possible to scan for conflicts on startup/usage and disable code gracefully.