You raise a very good point. All the current uses that have been tested—not all have been approved—for which some evidence exists of a beneficial effect of cannabis, it is not the most effective, the most potent, or the most reliable drug. However, some patients don't respond even to the best drugs, and for those patients cannabis can be a useful fallback.
A number of studies show if you combine cannabis with the other drugs, the more effective ones, you can get a better effect with less risk of side effects by using a smaller dose of each in combination, than a full dose of either one alone. That has been shown for relief of pain by the administration of an opiate drug together with cannabis in lower doses. It has been shown, in one study at least, with epilepsy where the conventional anti-epileptic drugs were combined with cannabidiol, CBD, to give a better control of the seizures than had been obtained with the conventional drugs alone. The possibility of combined use needs to be further investigated.