I observed that "by definition" is on its way to becoming one of these emphasis phrases, like "literally", no longer literally meaning what they used to.

Yes. Though even in its literal meaning 'by definition' is a weaker statement than 'by evidence'. At least in any argument outside of mathematics.

Most of the time the 'by definition' argument just gives you a True Scotsmen.

Take the classic: all men are mortal (by definition in Ancient Greek), Socrates is a men, thus Socrates is mortal.

As an empiric argument, this would make sense. But the Greeks tried to reach 100% certainty. Mortality was included in their definition of a human (otherwise you'd be at least a semi-God or so.) Of course, that just shifts the uncertainty to the second part: we can not be sure that Socrates is indeed human by that definition, until he actually keels over.