Why is it acceptable sometimes for writers to enclose an independent clause--a clause which could stand by itself as its own sentence--in dashes and parentheses, yet this can't be done with commas? An example sentence:
- "She won the raffle--this had never happened to her before."
- She had won the raffle (this had never happened to her before).
- She had won the raffle, this had never happened to her before.
Sentence 3. is clearly a splice, yet the comma is functioning in the same way as the dash is in sentence 1. (I apologise for the poor example.)
On a slightly separate note, is the em dash ever functioning in the same way as the colon? I know that in sentence 1., above, a colon could replace the dash, and therfore they are interchangeable, but does the dash always only serve as a parenthetical piece of punctuation or can it function like the colon?