Sure, but if we could paste HTML between editors and browsers without any rendering or layout issues, there would be no reason for users to look beneath the surface. I do agree that we're not quite there yet.
There is still the need for destination system to fully validate the input to catch badly formed content which may cause unpredictable issues and (more importantly) to screen out injection attacks, which may not be a trivial task. Relative to validating markdown decorated content it is far from trivial.
