With regards to rail connectivity:
You will find that LGW also has a fair share of commuters using the fast direct link from there into the city.
Other than that, MAN has no mainline services, whilst e.g. BHX is a mainline station serving many parts of the country. So it would be a feast for MAN to get to the same proportion of rail pax as BHX. A further platform may be beneficial, but doesn't address that it is essentially a terminus.