Pinworms usually don't cause too much of a problem unless the rabbit has a heavy worm load, then it could lead to reoccurring digestive issues, weight loss, lack of appetite, and/or poor condition.
Medirabbit: pinworms
The benefit of treating would be preventing a worsening of the parasite load and associated digestive issues. Side effects most common from giving the med may be some minor reduction in appetite. In more severe cases, dangerous diarrhea could occur and loss of appetite. In heavy worm loads, the worm die off can release excess toxins making a rabbit sick. And rabbits with compromised kidney and liver function may not be able to safely process the antiparasitic.
I would want to treat it even if it is just a light worm load. I can't see any reason not to, provided the bun doesn't have preexisting kidney or liver problems. Pinworms can be difficult to clear up, so a minimum 5 day course then one more day 10-14 days later, or a straight 10 day course, is what I'm seeing recommended now. Also a thorough clean of the rabbits environment.