Is there a point at which radiation therapy for cancer doesn't help enough to make the side effects worth it?

Yes. You don't always know if a treatment is going to work on a patient. If the cancer is too advanced and there is little hope of helping the patient the doctor needs to tell a patient and their family so they can consider hospice or comfort measures. Many times radiation actually is a comfort measure and given so their is no side effect but relief of the symptoms.
Yes. Like anything else in life, there is always a point whether or radiation of treatment can do more harm than good. This is a balancing act/ decision that each day radiation oncologists and their patients make before recommending/ undergoing a treatment.
Stopping radiation. Absolutely; like any medical intervention, it is not worthwhile if the odds of serious side effects outweigh the potential benefits; but in most situations the side effects are temporary and the benefits can be long lasting, such as pain relief, or increased odds of long term cancer control or cure.
Risk/Benefit ratio. Radiation like any medical treatment, has benefits and side effects. For incurable cases, radiation sometimes is useful in relieving symptoms (palliation) such as pain, bleeding or obstruction from cancer swelling. However, if the risk outweighs the benefit and the quality of life is compromised, that it should not be given. This has to be decided by patient, family and doctor.
Yes & no. There is a time when attempts to treat cancer needs to shift to the attempts to find comfort measurs. Radiotherapy done well, rarely causes substantive side effects when the goal is symptom relief. It all depends on your doctor and you to iron outif the adverse effects exceed the potential benefit.