I do consider Barrett a 10 (But a utility 10 rather than a specialist 10), and certainly a better 10 than a 15, though he is good in both places. I also think he would be handy on the wing.
Do you consider Barrett the best 10 in the country at the moment? If so does that make him a utility 10 because he can play somewhere else, and therefore not worthy of a start?
Was Dan Carter a utility 10? He played quite a bit at 12 during his career.