Our dentist, whom I've been with for over 20 years now (and MrsC several years before that) would have preferred to stay with the NHS.
The crunch came when one of the partners retired and they could not replace him. They had one applicant who was full of 'wanting to support the NHS' at the interview, but who then took a job with a private practice anyway. As a result, they felt they had to go private and are now part of Denplan.
From our dentist's professional point of view (I assume there were financial advantages as well) being private has meant he is paid to do the sort of job he always wanted and tried to do when with the NHS. A small piece of remedial work now rather than a bigger piece of work in the future. Effectively he's on a fixed price contract rather than time and materials for each filling.
As for hygienists, his comment to me when he went private was, 'you don't need to see the hygienist, but it won't do any harm and having someone who knows what they're looking at checking your teeth four times a year will catch any nasties earlier rather than later. The last time I saw him, he told me that, much like Helly mentioned, getting rid of the scale can prevent some gum disease, particularly as we get older. Dental treatment of older people is a bit of a new field as so many more of us still have quite a few of our own teeth well into later life.