Recently, I went through the trial by fire also known as "edits" with a fabulous editor on staff at Enspiren Press. In the period while we worked together, I learned my publisher would allow only a few instances of the em-dash or ellipsis. This was something that bothered me because I discovered I am ellipsis-challenged. My book was full of them.
My publisher prefers not to allow many uses of them because it can make your character seem dumb. What? Yes, that's right. Or at the very least, make them seem ignorant of facts.
Think about it. If you were talking to someone and they constantly trailed off a sentence, what would you think? You'd think they didn't know the answer, or they were hiding something.
But they are useful when used properly, and when used minimally.
Here from Scribe.com is a pretty decent description of what these little dots are supposed to be for:
The ellipsis is a series of three — and only three — full stops used to mark missing words, an uncertain pause or an abrupt interruption. For example:
The review said, "It's wonderful ... a complete triumph".
Niles: But Miss Fine's age is only ...
Fran: Young! Miss Fine's age is only young!
Most editors precede the ellipsis by a space, even at the end of a sentence.
Note: Within Microsoft Word the ellipsis can be typed as a single character, rather than three separate periods, by typing Alt-Ctrl-period.