Typically, a failed login is supposed to direct to a second login page that contains a static error message, so the process is the same as using a single login page with validation and a dynamic error message.
Technically, doing it all on a single page *is* more efficient in one aspect, it could be that using two pages actually requires fewer overall lines of code. And either way, the page has to be submitted so the only difference is which page displays when the login fails.
There are also other types of login pages, such as when someone tries to access a restricted page from a bookmark and is not logged in. They get directed to a login page which will need to display is own message and pass/fail redirects. so if they were to update the SA login behaviour as requested, they would need to make it to include all the scenarios, not just the one this thread is about, and that could get more complex.
I'm all for upgrades and more features, don't get me wrong. Let's just put our heads together to make sure the feature requests are thorough.