Actually, that is not true. Oral fluoride can help strengthen the enamel of your teeth and help strengthen your teeth. Fluoride is actually incorporated into your teeth (and bones) when you are growing.
Yes my dentist says the same, for growing children, this is the case. We do not have flouride in our water, and I didn't even realize until dd6 had a few cavities. He prescribed tabs for her since she is still has many baby teeth, however not for my son since most of his are out, at age 12. Flouride in toothpaste is good enough for him, he said, although for me I was switched to a slightly higher flouride toothpaste since I got a couple cavities since moving here, too.