Is there a legitimate reason for dental offices to insist on x-rays every six months?

Health of mouth: The frequency of x-rays is determined by your history of dental restorations, the amount of decay you have, your medical history, your diet, your home care, the amount of plaque and bleeding. Some patients with high risk of decay do need radiographs every 6 months and some patients can go 12 to 18 months. Talk to your dentist and discuss this recommendation for x-rays.

Yes: Small carious lesions don't always show up on a radiograph... Sometimes it's the angle that the film is taken, sometimes there's not enough decalcification, but it is still there. Six months later, it just may show up and then your dentist can diagnose and restore it. You can refuse the films, but if you end up needing root canal therapy, don't blame your dentist.

Recommendation: General recommendations regarding the frequency and number of x-rays have changed over the past year or so & resulted in a decrease in number and increase in interval of time. However, this is only a general recommendation and left to the discretion of the dentist based upon caries risk assessment for each individual. A dentist can insist and you can refuse, but that may or may not be wise.