The only possible reason I could see for using rubber ammunition is if you live very close to your neighbors, i.e., town homes, apartments, closely spaced houses, etc.
Rubber may be (or maybe not) lethal at down the hall or across the room ranges. People have died (although it's not that common) from different types of less lethal rounds fired from shotguns. But by design, rubber buckshot will slow down quickly with distance and punching through layers of drywall. So, you have less of a chance of serious injury to an unintended victim in the next dwelling.
Rubber buckshot is designed to be fired at people, although at longer ranges and after a glancing blow off the ground. Lead, being much denser, will maintain it's momentum far longer after punching through multiple interior and exterior (wood & siding) walls. Basically, lead stays lethal at far longer ranges than rubber will.
Other than that, I agree, not the best choice, although if it's all you have... As others have said, if you need to pull the trigger it is (or should be) a true lethal force situation. If someone thinks they will bounce a few rubber pellets off of someone and scare them away, that's going to end badly one way or the other.