Daynehold the suttles exist because of the exisiting routes ex:LHR, i.e. Air New Zealand, so good business sense to connect to them.
There is quite a good LH market from MAN as evidenced by AA/BD/SQ/EK, etc. I know the demand for ORD from MAN is strong and bmi's Las Vegas service does really well. I do think there is enough demand for a direct LAX or SFO, that would be a brilliant point to point and also great for transferring pacific pax.