I'd say so considering you only have access to the current download for Sonar at any one time.
I think rollback is possible/with reservation, even if the files are not kept local. Noël assured us that at whatever level you buy in, those files will always be available for access. I think it's complicated though. If I'm on Cambridge and its stable why would i want to roll back to Allston or Braintree. I would find that out at time of installation. The community very quickly finds out whether a release is stable or not. I can't find a logical reason to roll back other than its mine and I want to.
If you buy in now whether it be for the year or month,, the installer is Cambridge not Braintree or Allston. It's clearly a stable version despite a few bugs of inconvenience but no show stoppers.. Once you're in the membership though there's no reason why you can't wait to update on the monthly releases if you want to. Each version update is rolled into the previous ones so there only ever one Sonar and that's the current one.
The version lines have now been blurred and we no longer will have definitive X2, X3 type releases.
There is Sonar Artist, Pro or Plat at whatever time you purchase. In a way I guess that Sonar has become a moving target as far as the precise Sonar executable version you will get.
That should apply to content as well. Eg The Brian Hardgroove loops that came out with Braintree I imagine are not included with any Sonar purchase now at Cambridge time. They will be available for purchase in the Cake Store at some later time we've been told.