Clearly, in the sense defined, it is very sensible to assume people have an unconscious, and Leibniz's simple argument for its existence is quite good: One may be woken up by sounds - hence there was something registering sounds that one was not conscious of while it was registering.

In other senses of 'unconscious' - for example: Freudian - much more tends to be assumed, which is mostly nonsense, since there is to date just no good scientific explanation of what consciousness is or how the human brain works.