Adobe had significant updates & changes in October 2013 and December 2013, but those didn't require a new app. Why now?
This update caused some major issues in our pipeline and we had to roll back and install previous versions of the apps during a tight deadline to get our deliverables out of the door... So I like the idea of "never touch a running system" and add the new version in parallel instead...
But as you write plugins also have to be considered here... By default the should work in the old version and you should be able to install them on teh new version to check if everything works fine...
If I think about Autodesk software there is the same situaltion: We install the new version 2013...2014...2015 in parallel and then adding all the plugins. If everything works as expected the next to last version can be deinstalled from the system.
So you have a stable sytem and a new system all the time...
--> CC2014 Apps are in approval stage now. We test them with our leads and if there are no show stoppers we set the signal to switch all workstations at once company wide...