My sense is, for many users, renaming a scene happens a lot more often than renaming an actor. Thus Cmd+R is Rename Scene (more frequent, less modifiers), and Command-Shift-R (less frequent, more modifiers) is Rename Actor.
But I would very much like to add the ability to define your own shortcuts for a future version. I'm sure everyone would like that. But, with all the other features added to 2.0, it was not something I could do for the initial 2.0 release. Hopefully a little ways down the road we can do it. Best Wishes, Mark