I agree that the shuttle is truly an amazing machine. Nobody denies it. But the shuttle costs 1.3 billion dollar per launch (if you include the shuttle cost and the costs of buildings, facilities, training, salaries, etc...) so for 20 people that is about 65 million a person. Are there 30 people that can pay that per launch? Where would they go in LEO? The shuttle (being side mounted) is constantly in danger of fatal TPS damage. What will happen if another Columbia type accident happens and 20 people die in one shot?
In short the benefits are not justifiable for the risk you are taking. As stated in the X37 thread you can learn from the shuttle and maybe design a smaller shuttle for just passengers that can be mounted on top of a rocket?? But the shuttle as it stands right now is only needed to finish its job with the ISS and then retire in grace. Besides the shuttle technology is over 40 years old now, its time to let it retire and take its place in history.