At no point do I suggest a mental addiction is less harmful, just that a distinction exists.

With a physical addiction, the substance itself becomes necessary to the body. A psychological addiction involves becoming reliant on a perceived effect of some other stimulus. If you give a brain dead person a physically addictive substance and then stop, they will suffer withdrawals while they can't develop a psychological addiction as there is no perception.

Generally, the two will develop together and both are harmful.