My answers in blue.
1. Why are the majority of western horses in shank bits? Why can't you just use a snaffle? From my understanding, most western horses are started with either a rope halter or snaffle.
Cori had it right, shanked bits are designed so that you can use almost invisible cues with the reins. What you can do with a smidge of pressure on the reins in a snaffle bit, I can do in a curb by only moving my hand a few inches and never tightening the reins. The shanked bits are what allow us western riders to ride on such loose reins and still have good communication.
But what about the horses that are taught to neck rein? Shouldn't it not matter what they have in their mouth or on them? My understanding is that when a horse is taught to neck rein they are taught to move off of the rein on their neck.
Using a shanked bit is about more than just being able to neck rein. It's true that the bit should not be effected at all when you neck rein but if you need to pick the horse up with his mouth for any reason, to either get them back to a level headset or if they begin to get strung out and you need to collect them again, then you can keep your loose reins and only lift your hand an inch or two to get your desired response. With a snaffle, you would likely have to shorten the reins enough to actually make contact with the bit.
2. When people say that they ride in a rope halter do they mean a bosal/hackamore or a plain rope halter? If it is a plain rope halter, how do you attach the reins? Do you just tie the leadrope?
That was an excellent picture example Poseidon, I will just tie the lead rope to the halter when I use one for riding. Most western riders don't use the term bosal unless they are for sure using a rawhide bosal hackamore. A rope halter with a mecate rein set (like Poseidon posted) is also sometimes called a soft hackamore or a loping hackamore.
Why use a rope halter? Why not use a bosal/hackamore? Couldn't riding in a halter cause a horse to think halter=work and perhaps make it difficult to catch them?
Because a rope halter is easy, it's fast, and most people have one handy. A good bosal hackamore will cost well over $200 and unless you know how to properly use one, it's more trouble than it's worth and pretty easy to screw a horse's nose and jaw up with one. Most horses who are actually used for hard work every day begin to associate just being caught with hard work and that is one of the reasons why they get hard to catch. My horses that are easy to catch in the winter get hard to catch in the summer simply because they are used for heavy work on a nearly daily basis.
3. What do you strive for in your riding? I know, that as a dressage rider, I strive for an even gate with impulsion and a even contact on the reins.
I strive for softness, relaxation, a level topline, responsiveness, cadence, and heart all on a loose rein.
4. Why do they tend to start horses earlier? I went to a AQHA sale and all of the horses that sold for any decent money were two to three years old and could neckrein, walk, jog, lope and one man even took the bridle off of his horse (a two year old mare, she was a sweetheart) and steered her by placing his hand on each side of her neck. Of course, I'm more used to horses being started at three the earliest.
With the AQHA, APHA, and ApHC futurities, horses are started showing at 2 years old. Most breeders to try for foals to be born early in the year (Jan, Feb) so that they are closer to actually being 2 when they are started. A foal that was born in say........July would have to be started under saddle at approximately 18 months in order to be close to show ready for the futurities it's 2 year old year.
That is an excellent video Poseidon. He is showing a quick and easy way to teach neck reining.