Yes, RhinoCommon is updated with each release as there are typically new functions/classes that are added with each service release. This is also the reason that “This plugin requires the latest build message” is there. When a plug-in is loaded, we look at which version of RhinoCommon the plug-in was compiled against. If that version is higher than the current running version, we display that message because we don’t know if you are going to be attempting to use one of these new functions/classes which don’t exists in the older running RhinoCommon.
If you want your plug-in to target a service release that is NOT the most current one, then yes you should copy RhinoCommon into some directory in your project and have your project reference that version of RhinoCommon.