Eh. This is DM fiat I don't need or want. I realize that I'm expressing a person bias, but some things (like "beat AC to hit NPC with sword") should be clear, consistent and fiat-free. I think that spells oft used by PCs should be in that category.
Consider example #1:
You're character is Bob, he's a Cleric. He can cast Raise Dead. One week your adventuring companion Mirt the Stinky dies. You don't like him much (particularly how he smells), but out of fellowship and duty you case Raise Dead. Because "he has a Destiny", it works. The next week a band of orc raiders sweeps through your home town and (mercifully) kill Bob's sister. You rush home to "save" her, but your spell fizzles. You cast it exactly the same as last week, but she just doesn't "have a destiny." Too bad.
This, to my mind, is a bad result. The PC should either be able to raise the dead or he can't. I guess I'd be fine with it either way, but this "only if the DM feels like it" rule just doesn't work for me.
Now consider example #2:
Your character is Bob. It doesn't matter what class he is. What matters is that he has spent the last two years (in campaign) tracking down the sorcerer that killed his family. He tracks him down to his lair and kills him.
Too bad! That NPC has a Destiny! No matter how many times you kill him, Clerics of the Raven Queen can just keep bringing him back.