The commercial pilots license was not created to sign a person off to go and fly a 50 seat jet at 250 hours. I believe that anybody can be trained in the simulator to fly any aircraft, but operating that aircraft on the line in revenue service is a different animal with a different set of concerns. The real world tends to come up with some pretty strange things to throw at you in the air and it is these times when falling back on actual operating experience will help to keep you from bending an airplane. Don't get me wrong, even with thousands of hours under your belt, you still stand a chance of something going haywire and we all need to keep on our toes and learn every day, but the odds of a successful outcome favor the experience over the inexperience.
Airline captains are not paid to be instructors, and having a person in the right seat who has a solid background and established skill set is important while operating in the system with 10, 20 or 50+ people behind you. I am sure that there are exceptional 250 hour pilots out there, but the odds are not in their favor and excpetions don't make rules. Obviously low time pilots can be trained to operate these aircraft to an established degree of safety, but I do agree with the statement that simply because a person can fly a sim in training, it does not instantly make him a super F/O. There is more involved than just moving the yoke around!
VT