My opinion is that in certain scenarios a tension driven subdivision of work may produce roles with pretty limited scope, the activity of which can be fully defined through a few accountabilities. In other situations a well defined purpose can work better than detailing accountabilities, and leaves more room for interpretation by the role filler. If we let go of the design upfront approach to role definition, we can easily accept that tensions will shape them in many possible ways, a role may even start its own existence with the ownership of a domain and no other specs.
I think this aspect of the Constitution is very smart, as it allows to produce an initial role configuration that's as simple as the situation requires. In the end the goal of holacracy is to set up the minimum amount of rules that are needed to run the organization in a clear and tension driven way.
I too agree with you that a roles needs somehow an implicit or explicit purpose to have meaning, but think about the existing organizational structures, do all roles have purpose there? Not in my experience. Holacracy must also give room to reflect current reality as a starting point to capture and evolve what's already happening.