Paul,
they told me the same thing and 15 months post Tx my teeth are still there and and no sign of ORN. Pulling the back teeth seems to be SOP at some treatment centers. I would get a second opinion. Some people consider losing their teeth to be the worst part of the whole experience. The risk of ORN is about 10% and some get it in spite of having their teeth pulled. They just want to avoid liability if you are not in the 90% that don't have a problem.
The ways that doctors are sued these days they have to strongly emphasize any and all risks no matter how small or absurd.
The dental oncologist first recommended pulling ALL of my teeth, then when I told him that it was only the molars they (the radiation oncologist) were concerned with, he changed his tune. I had a panorex x-ray done by them. Then I went to my regular dentist had bitewings, repairs, cleaning and impressions made for dental trays for the in home flouride treatments. I go in every 4 months for cleaning, also use a WaterPik and floss at least once a day. I have had 2 small repairs (fillings) since treatment. It will require a lot more work to maintain your teeth. It may sound crazy but I did much homework on what path to take with this and I opened up the bible to a scripture verse that said "...the Lord will protect all of your bones" - that was what tipped the balance in favor of keeping my teeth.

You need press your dentist for the truth about the real condition of your teeth and also let him know the risk you face if he is not rigorously honest with you.

So many tough decisions - so little time to make them...