I agree with you, on some points. Some specific dates are important because they've been seminal turning points, others seem a bit too specific, like say, the end of the Western Roman Empire. The specific date of 476AD tends to make people think the Western Roman Empire ended in a bloody, fiery revolution in that exact year, whereas the Empire just slowly, and mostly peacefully, dissolved over the course of decades.
Important dates when things really change abruptly, like say the French Revolution (even then, it makes people think that atitudes change overnight, when they do not), are important to remember as turning points. But something as abstract as an "era" like Classical or Medieval is impossible to pin on a specific date. Even just defining these eras is an impossible task, let alone determining when one begins or ends. The concept can be useful for basic organization but the problem comes when people say "oh, that could not of happend then! Its not medieval of them!" or whatever.
anyway, i think we are getting offtopic