It is not inadvisable to drive your car with low pressure because it creates lots of troubles on the road. I am sharing my personal experience here. With low tire pressure, my car had been less responsive and was giving less traction. Therefore, it became difficult to avoid any emergencies. With less traction, the car engine has to work harder, using more gas than needed. So, it puts pressure on the engine and tire wear, fuel economy, and safety of all. You already know about the effects of driving with low tire pressure, so it doesn't make any sense to drive at low pressure. But, in case you are wondering how low, it's 20 PSI. If the tire pressure goes lower than that, it's considered a flat tire.