Tooth with crown sometimes throbs with my puls, mostly at nighte. doc wants root canal. reasonable? xray shows small dark area.

Absolutely: It appears you have an abscess at the tip of that tooth, secondary to an non-vital nerve. Failure to treat with root canal therapy will result in a larger abscess, continuous pain, and swelling. In the end you will either have the root canal therapy or extract the tooth.

Root canal needed: The little dark spot is an abscess, which is an infection, which is caused when one or more of the nerves die in a tooth. You will continue to have throbbing pain and pain to just about everything else until it is root canaled or extracted.