There are a lot of people doing pure lines. However, crossing for a nice trait is also common. Subspecies cross is something I personally don't like, but many people do.
The twitching comes from a neurological disease from all the genetic mutation for the jag trait, a double recessive gene. Most with obvious twitch are euthanized. There is also a symptom of that snake being laid on its back and not being able to flip back over. Those are also put down in most cases.
Just a side note, your post was very difficult to read because of the lack of punctuation.