Magplug - MAN's taxiway and stand infrastructure would never enable 05L/23R single runway to achieve Gatwick runway movement figures at peak times. Flexibility at holding points is limited therefore difficult to achieve best departure sequencing. Runway exits may not be ideally positioned for arriving aircraft to minimise runway occupancy times. Some parking stands very close to runway causing congestion etc etc. MAN's taxiway layout looks constrained and complex therefore ATC need to be certain pilots fully understand which route they have been cleared. I'm sure any wrong turn must cause major headaches for GMC.
One big attraction of a second runway is that in the event of one runway not being available the airport can continue operating, albeit at reduced capacity. This means a lot to the big carriers who look very closely at contingency in the event of an unplanned runway closure. What better to have another runway available. That's why LGW has a standby runway, maybe not the same as the main runway but over many years it has served a purpose.