In the latest versions of RA, more and more functionality has been streamlined so that the vast majority of tasks can be performed through ROC if one so desires. However, a few specific features are still only available through Automation Studio, which means it's likely that at some point during application development to deployment, you'll have to hop back and forth between ROC and Studio.
A prime example is setting up Test Agents. If you're designing a process in ROC, in order to test the process, you'll eventually get a pop-up message advising you to go into Studio in order to assign test agents. I think it would be more efficient for users who prefer ROC to add that functionality directly to ROC, under the Administration menu, where other agent management features already exist.