This is an interesting topic which I encounter in my research on UX integration as well. The results of my studies in close collaboration with software development companies clearly showed that there is a lack of agreement and low awareness about UX related roles and responsibilities.
Although I agree with the previous answers where what a UX person takes on differs from one organisation to another, or one project to another, I relate this to the field of UX being relatively young. This however creates freedom in such jobs, ends in a number of challenges for UX people as well. Many non-UX practitioners do not have a clear view of UX roles therefore might have wrong expectations when hiring people. On the other hand, since the role of UX person might overlap with some other roles (e.g .requirement analysts) is it even more important to clarify the role and its responsibilities.
Today, if somewhen says 'I'm a software architect' at least people in the field get a sense of what he/she might be doing. The same for 'web-developer', 'requirements analyst', 'product owner'. In my view, their roles being better defined and established makes their job in the organisations 'smoother'. This is while one of the common problems for UX people is to justify their responsibilities.
All in all, I agree that we should have a classification/list of UX related roles and their responsibilities. This however does not mean that a UX person will not be able to do anything else if required.
Going back to the 'architect' example Stewart gave above, everyone knows what a architect does for a building, but if required (e.g. in small projects) they can perform some other activities that are often performed by other roles.
So far in my research, I have seen spectrum of 'skills' and responsibilities for UX people: at lower level (e.g. prototyping, graphical design, interaction design), at middle level (e.g. information architecture, requirement analyst), at higher level (e.g. solution builder, business analyst, product vision and strategy, research and development)
Based on my experience, I believe a 'UX expert' (don't have a name for it yet) should have the most contribution in the higher level that deals with the strategy, vision and does not go into details of design.
This means such an expert should have more authority and decision power compared to a usability test person, or interaction designer. (s)he also needs to be aligned with different people at higher levels (e.g. subject matter expert, product owner, product manager).
But still many organisations only replace the term 'interaction designer' by 'UX architect' without necessarily changing the authority or responsibilies of that role.