Historically, letting the browser guess anything at all has been a disaster. Although IE is all but dead now, there's no guarantee that another browser won't try something similarly reckless in an attempt to be a little too convenient.
So i guess the question is — should we keep markup for the case where someone downloads our webpage on a legacy browser?
For me the answer is yes, if people are using an old netbook on a dialup connection and they save your site locally to save data. That could be a good idea. I'll do some testing though because I think even old IE defaults to utf-8.
The edge cases are hard to test. IIRC the weirdest issues had something to do with valid UTF-8 content the first X bytes of which also happened to be valid in some exotic charset related to the user's locale. English users would rarely come across such issues.
Historically, letting the browser guess anything at all has been a disaster. Although IE is all but dead now, there's no guarantee that another browser won't try something similarly reckless in an attempt to be a little too convenient.