Dental examinations, even with enhanced lighting and magnification, often do not reveal everything. Large cavities and bone loss from gum disease that start between teeth are revealed at the end of the roots and detected by x-ray. Hollowing out of the inside of a tooth can take place with little or no effect on the enamel.
Decay detecting x-rays as a regular practice should be taken yearly. Those with low decay rates can decrease frequency, keeping in mind that some things may not be detected solely by visual exam. Radiation exposure is very low, equivalent to a normal day of indirect sunlight exposure, and is of little consequence in comparison to the damage that can be done when decay goes undetected.