Broad acceptance? I'd say it is rather narrow acceptance, albeit universal within a small but dominant group! I don't think I'm the only one who prefers the use of all caps to indicate emphasis, rather than italics, bold face, or underlining, for the simple reason that it can be done in plain text without resorting to markup language. But the catch is that it has to be minimal. In a pinch, I'll go for *asterisks* or _bracketing with underscores_ (am I bringing back some memories -- or nightmares -- for old BBS users?), but I'd rather not rely on hidden codes to convey my meaning in a public discourse.
Sorry, but for me, they are not easier to read -- italics, underlines, and bold are a much more readable way of emphasizing words, and (where available) should IMHO be used to the exclusion of all caps, except for the conventional usage to actually indicate shouting.