The molar on which the brace rings were to be fixed had a cavity. i told this to my dentist but he neglected it and said that since molars will be covered, there was no need to fill it.did he do the right thing?

Cavity: If the cavity was very small and shallow, the band may be covering it and the cement may seal it. However, i would most likely want to remove decay and place a filling to cover and strengthen before the bands and brackets were attached. I would like to be sure that it would not continue to decay and become a much bigger problem.

Not true: If that is what he said / meant. You can have a cavity on the chewing surface of that molar, that can be filled even after the bands and brackets / wires are placed. Does not interfere with the procedure of the filling. If the cavity was between teeth, then the cavity cannot be adequately filled. To neglect the cavity altogether is not advisable. Perhaps a second opinion might be advisable.

I'm not completely: Sure either what your dentist said nor the interpretation. It's difficult at best to render an opinion without having the opportunity to actually see both the tooth and the x-ray of the tooth. However, i personally wouldn't and don't agree with not repairing or filling any cavity, hole, decalcification, etc. On permanent teeth, nor primary (baby) teeth unless they will be lost soon.

Danger: A cavity is an infection. No orthodontic specialist that i know (are you seeing a specialist) would put a band onto an infected tooth. Covering an infection does not prevent it from spreading. Have the band removed, the infection eliminated, and then the band placed back onto the tooth.

Disagree: Get a second opinion. No brackets or bands should knowingly be placed over an area of decay.