FireLance said:
Getting a little difficult to choose, but I've decided to go for quality over quantity. I'd rather have one more accurate attack than two less accurate ones, so I'm going with Rapid Shot. By that reasoning, Two-Weapon Fighting will be next.
That's a valid reasonning when your opponent is tough to beat, but still in your range -- if you need a 15-20 on your attack roll to hit, say. (Warning: actual number crunching are left as an exercise to the reader.)
But if you're facing someone who's really tough to hit (need a 19-20 or, worse only a natural 20 can hit because the AC is too high), if they have concealment or invisibility, or if they have an AC that is so low that you'll hit anyway...
Then it's better to have more attacks. For enemies that are tough to hit (either because of insanely high AC or because of miss chance from concealment), the more attack rolls, the higher your chance one of them will hit.
For enemies that you will hit anyway, more attacks means more damage -- in that way, TWF (and Rapid Shot) becomes a form of Power Attack that deals double damage for just a -2 penalty on your attack rolls. It's worth it -- much better than Power Attack, actually.
In fact, the reasons to use TWF or Rapid Shot are the same as the reasons to use Power Attack. "I need a nat 20 to hit? Screw that, I take -10 on my power attack, that way, if I do roll 20, I'll inflict 20 more damage!"
Except TWF is better because, contrarily to power attack, you can choose to deal the extra damage on somebody else... With Power Attack, you need Cleave for that, and it'll only work when you get a kill.