Pam Peters writes in _The Cambridge Guide to English Usage_ (2004):

>_Ought_ seems to have reached the end of an evolutionary phase in which it might have become a fully fledged modal. But the trappings of its older identity as a lexical verb have hung around — in the fact that _to_ is almost always there to link it with the following verb, and in the use of _do_ support in negative statements. So while _ought_ still works affirmatively as a marginal modal expressing obligation, it is otherwise replaced by modals such as _should_ and _must_ in non-fiction writing of all kinds, everywhere in the world.