If yes didn't exist then man would never evolve and nothing would ever get done resulting in the eventual end of humans.
Without no we'd all just be a little 'slave-like', but at least we could answer back to defend ourselves with yes "Would you like to give me your house?" "Yes, but would you like to not have my house more? Yes" etc etc and so on.
So, as DF said... Yes is far greater than no.
And saying no to everything doesn't make you a rebel.
