1. A high keyword density for the phrase you wish to rank for is very important to achieve your desired ranking.
2. The more content you squeeze onto one page, which could be spread across multiple pages, means the keyword density will be lower.
3. The page will not rank as well as it could have. It's basic SEO. Maybe dentistry isn't a very competitive niche and you can get away with a lower keyword density in the body text.
1. I can beat keyword density out with a good title tag, 2 related headers (h1 & h2) & some inbound links.
2. True, but see my first point.
3. True, BUT only in a competitive market. In my original comment I said "...can be great for a small business.", I probably should have used the words "local small business".
Because ranking for something like... "Miami Electricians" or "New York City Dog Sitters" would be easy and would be most effective (in my opinion) with a one page site.
A good title tag needs to focus on one or two key phrases. If the whole site is on one page, you can't make a good title tag to cover everything. Better to have a different page for each topic and optimize title, body text, and thinks for the topic the page covers. If all inbound links are equal, the optimized page will always beat the all-in-one page.
2. The more content you squeeze onto one page, which could be spread across multiple pages, means the keyword density will be lower.
3. The page will not rank as well as it could have. It's basic SEO. Maybe dentistry isn't a very competitive niche and you can get away with a lower keyword density in the body text.