Well, I think both arguements are stupid. "A horse needs training to be ridden"? Yeah, um, some people train in bosals, sidepulls, hackamores, snaffles, mullens, curbs, and saw bits. Training isn't exclusive to snaffles, so that arguement is void.
And the other "horses take advantage of you and no training can fix that so you do what you need to do to fix a willful horse" makes no sense either. First off, what is training for, if not to "train"...? And horses take advantage in any bit. They can grab the snaffle and run away, and they can grab a curb and run away. They can throw their head up to avoid the bit and then can tuck their chin and evade the bit. So this argument is also void.
Now, I think your issue is that you seem to think the bit makes the horse. So without a bit you can't ride your horse? Are you one of those 'hands-y" peole constantly fiddling with the bit? What about seat? Leg? Balance? Voice? Posture? Whip? Spur?
Moving on, what discipline are you? Western people believe in reaching contact through the neck, not the mouth, which means curbs are ok since the only time they take contact is when the horse needs correction. You see, a curb bit is a punishment bit. When the horse takes contact, pressue is applied to both the underside of the jaw, bars, and tongue. Depending on the bit, pressure is also applied to the poll. All of this discourages contact, which is why it is such a good bit for western disciplines. Those horses rely on neckreining and seat cues, not a connection with mouth to hand.
Now, if you're a dressage rider, it's a different story. Dressage riding is all about contact. (And yes, the upper levels use shanked bits but most are mullen, and just because they use curbs doesn't mean they all use them correctly, and a true dressage horse goes just the same in a plain snaffle). The horse should have self-carriage but should also have a light contact with the rider. Curbs discourage this contact for all the reasons listed above. (Increased contact equals increased pressure, equals increased discomfort). What I also dislike about curbs is that any movement is translated to both sides of the mouth, whereas with a snaffle you can hold the outside and open the inside and the horse will be able to follow it because the bit has so much movement.
Again, if you can't get the same results in a halter, or sidepull, or snaffle, or mullen, as you can with a curb, you aren't really doing it right. A trained horse will be effected very little by a change in bits. It should know its job and be able to perform it whether it's in a snaffle or a pelham or a kimberwick. Now, it might go better in a certain bit, but that's another story.