There is not only a business case for a parallel taxiway there is a safety issue as well. MATS 1 states that an active runway should not be used as a taxiway, but this happens all the time at MME. ATC has to increase separation on final approach to allow for this.
In terms of a business case the future plans for the airport include a parallel taxiway. This linked to the fact that airlines would prefer a parallel taxiway means that there is business case for it. Currently anything over the size of a 737-800 has to backtrack and quite often smaller aircraft will backtrack due to the conditions of the taxiways. A parallel taxiway would solve all this and make MME a more effficent airport and more attractive to the airlines.