I work professionally with other people's writing, and it's becoming clear that even many educated people have no real idea any more of the distinction between a colon and semi-colon. I think Lynne Truss's
Eats, Shoots and Leaves gets it about right when she says that it's down to technology. Texting and emailing on a handheld device are making people feel unsure about the distinction - not least, because they both look just the same on a mobile phone screen! A long dash can bridge the gap.
And Truss agrees, a little grudgingly, that there's nothing particularly terrible about that, because language needs to change along with our needs, just as long as we remain intelligible to each other when we express ourselves in writing. And that, although it's best to stick to the standard written usage if you don't know your reader, new stuff like @ signs, bullet points (possibly the best grammatical innovation of the last half century?) and now hashtags are all perfectly logical in their way. So she's not quite as zero-tolerance as she sometimes makes out.
WRT colons and semi-colons, I like to think that they're like the Stop sign and the Give Way sign at a road intersection. The first says, "whoa, there's something important about to happen, probably a whole new main clause, so pause and take a breath before you advance." Whereas the second says pretty much, "no real need for a full pause here, but be aware that you're joining a new section of road." A long dash can cover both eventualities.
BJ