Tim , why shouldn't a low houred pilot operate in the right-hand seat of a 737 ? These days , despite the hands-on training for your CPL skill test & subseqent IR , the majority of actual line flying is really systems monitoring . There is a lot of value in having wrestled the controls of say an F27 down the ILS at Jersey with a 40kt x-wind in moderate turbulence with a 60ft cloud base & RVR of 300m (yeah right !) , but in reality typical airline flying today is just not like this anymore .
After all , if the inexperienced lacked the required ability , BA , bmi , etc wouldn't be putting guys & girls into the right-hand seat so soon after graduating . It's all about aptitude .