It may be "clearly one-per-page" now, but it may not be. You don't really gain anything by using an ID. Keep the selector specificity simple so it doesn't bite you in the ass and cause a refactor.
.page-wrapper
.page-main
%article.main
There's no difference using that to
#page-wrapper
#page-main
%article#main
Psuedo example, those can be mix and matched with classes and IDS but doing so just complicates the selectors and the cascade.
There is of course exceptions and things that may just make total sense, but overall, it leads to complications. You don't gain any real benefits unless it's a JS hook for #id selector performance.
.page-wrapper .page-main %article.main
There's no difference using that to
#page-wrapper #page-main %article#main
Psuedo example, those can be mix and matched with classes and IDS but doing so just complicates the selectors and the cascade.
There is of course exceptions and things that may just make total sense, but overall, it leads to complications. You don't gain any real benefits unless it's a JS hook for #id selector performance.