For a competent team, arbitrary dates, generally leaving not enough time, only serve to limit scope. A competent team will produce as fast as is possible, given a specified scope.
Features are done when they're done (for a given definition of "done"). Once we're satisfied with what we have, we release it. Otherwise we keep working, or we scope down.
Strict deadlines are seemingly based on financial period expectations, rather than on what actually produces better software, and thus, ironically, more revenue for the same stockholders who were demanding a date. It seems more of a short-term strategy than long-term.
Anyway, that's my pessimistic, armchair interpretation of it.
I know you specifically say "for a competent team", but I just wanted to mention that for a lot of teams, the quote above is absolutely true.