I was taught that the semicolon is best used to
separate main clauses with contrasting ideas --
 
He went swimming; she did the dishes.
 
The colon and dash, I was taught, are used to
separate main clauses in which the second clause
amplifies or specifies information in the first (the colon
being more formal than the dash) --
 
He drank a lot -- he had three vodkas and eight beers.
 
Are these "rules" still accepted and being taught, or
am I an old fuddy-duddy?
 
Also, I recently shocked someone by saying that "ain't"
will eventually be accepted as acceptable English and
that "Winston tastes good like a cigarette should" will
become standard usage.  Your comments, please.
 
Thanks,
Ed V.