Yes, it happens. No it doesn't happen very much. I have no idea what a figure would be, but I think I'd be safe to say it's probably less than 1% and probably way less than 1% of dentists who graduate who either immediately or after a while decide not to become full time dentists and settle for being hygienists. I was the one who mentioned it in the last thread, and only because I know of two hygienists in San Diego who are actually DDS. That's all I know of, and there are well over 2000-3000 dentists in San Diego...not that I've met them all. I also know some dentists with very small practices who do all their own hygiene appointments so they don't have hygienists. They also do the other dental work too, but they do hygiene to avoid paying a hygienist, which is expensive, and so they don't need to have as big of a client base in order to keep the office running all day.
The one hygienist who I've spoken with more about it graduated sometime in the early 90s and knew right away she didn't want to do everything in dentistry (not just having to deal with setting up an office and such, but also didn't like doing fillings or crowns, etc.) She took a job in a perio office and has done cleanings and root planings for the last decade or so and says she's perfectly happy, makes great dough, and will continue doing this as long as she likes. Although a search on how much a hygienist makes an hour or monthly varies greatly...it is generally higher than what things like the OOH, salary.com, etc. say. According to those sites, average hygienists make roughly $25-30 an hour, and have average yearly salaries around 50-65K. In San Diego, where I live, the dentists I speak to and the ads for hygienists I see usually say about $350-$500 per day, which is more like $50 an hour or 100K a year.
You don't have to go into hygiene, though, to avoid the business side of dentistry if that is all you dislike. I'm sure more than a few dentists wouldn't mind an associate who stayed an associate for more than a few years without trying to break out on their own. You could just stay an associate without ever becoming the lead dentist in your own practice. You could work for a dental corporation or be a salaried dentist at a dental clinic that serves a specific population. All of these positions pay less than the average private practice dentist who owns their own business, but it's not chump change...at least $100,000 if not more.