Charlotte,
As Lilith said, some don't like the vest, for one reason or another. I know emily doesn't like it either.
My husband on the other hand, LOVES his vest. Not only does he feel that it clears him out better, but he can do it on his own. Quite honestly it was a pain for both of us to get up 1 hour early to do it in the AM. Not so much for him as he stayed home and did school work, but for me because I was working 12hr shifts at a hospital for a while. And then to make time to do another hour in the evening, it wasn't easy, not at all.
With the vest, he does 20 minutes in the AM and 20 minutes in the PM. Plus he does his pulmozyme at the same time, so he's reduced his treatment time.
When he isn't feeling well, he can bump it up to 3 treatments a day.
I think it's good for the research reasons that amy mentioned, plus to be honest, the convenience of it.
I don't quite understand why your doc. would have a problem with it. It's not like they have to shell out the money for it. I do recall it took a bit of paperwork on the part of Mark's doctor.... maybe that's what the problem is. Either way, I think a doctor should at least let a patient try it out.