The Pros and Cons of Prophecies and Destiny in Your Story (The title of this post reminds me of fractions. >.< ... What did it remind YOU of?)
PRO: These two can add an overview of your plot. You know what to expect, because it's right there, expressed in writing from some wise old nun or something. Almost like a brief, overall summary. Example: the Gregor the Overlander series by Suzanne Collins (same author who wrote the Hunger Games). But it can also be stretched to your entire series- such as the Warriors saga by Erin Hunter. "Fire alone can save our clan" and all that.
CON: It is very, very overdone. Sometimes it seems like any fantasy novel you stumble across has a prophecy in it, or the characters believe they're doing it because it's their destiny. Like the Leviathan series by Scott Westerfeld. I love, love, love that series, don't get me wrong, but Aleksander is convinced that all of what happens to him is destiny. Or the Percy Jackson series by Rick Riordin- Percy is a pawn of some sort of prophecy or other.
PRO: It does add depth to the story, if you do it right. Again, I bring up Leviathan or the Warriors saga. Your characters are focused on something, and their reaction to this destiny/prophecy (do they fight it? Do they even believe it will come to pass?) provide detail to "flesh out" your characters, as other bloggers on here would say. :D
CON: It can make your story seem gaudy and overdone. It kind of ties in with the other con; it's kind of like vampire romances, you want it to seem original and realistic. If your character is convinced it is his destiny to fling himself off a cliff and he does so, your story ends pretty quickly. If your character scoffs at the idea of jumping off a cliff being his destiny and is desperately struggling against it, and he eventually jumps off anyways, two things are gonna happen: 1. your character will seem a little stupid. Or weak. and 2. You just wrote a completely pointless story. I usually find it's easier to stomach a book where the characters are fighting destiny, which is just my opinion. This way, if your character's destiny can at least prolong your story for more than two sentences. But back to my main point: you know your destiny/prophecy is overdone or gaudy when it is dramatic, pointless, and/or all the character thinks about.
Prophecy and Destiny are good in moderation; I've come across plenty of books with these in them and they're among my favorite reading material. But if you overdo it, you might as well bang your reader's head into the desk for them.
P.S. - if you dare write a vampire romance about a prophecy where your human and vampire are destined to fall in love, I will not be picking it up at my local bookstore. No offense.