I don't think people see him as a specialist centre, more that he is a utility back with a strong preference for OC. Just because a player can cover many positions, doesn't mean they don't still have a best position and should ideally be picked for that. Daly is more able than most at covering multiple positions. Eg Farrell is not a 12, he's a very good cover for 12 if needed but is not a very good starting 12. Lawes was a good cover for 6 but was not a 6 until quite recently (Even then I think there are better options). Daly was a step further in that his ability to cover positions was even better but I think realistically, he has always been a 13.