One of the advantages for airlines using British airports is that airlines know when one of their aircraft land how much they will owe the airport. Metro's activity fee system makes this calculation far more complex. The precise cost of landing is not known until the end of the calendar year at which time adjustments are made. In 1986, the airport fees were 50% too high and resulted in fee adjustments in 1987. Had the fees collected been too low, a large cash payment would have been required from the airlines to make up the revenue shortfall. This may create accounting problems for the airlines.
Although not a large portion of their costs, this uncertainty over fee levels makes it difficult for airlines to project costs and adjust revenues accordingly. The entry and exit of carriers from the Detroit market can radically affect the landing fees at any point in time. For new carriers considering the use of Metro, fee projections become an important, but unknowable, variable in determining their cost of doing business at Metro.
Certainly, the airlines and airport incur a significant accounting cost in determining the landed weight, activity fees and airport operating expenses for which the airlines are responsible. To date, the airlines have not taken the time to audit the costs at Metro (perhaps because of the time and expense). A fixed fee system would eliminate the need for much of this accounting. 
Additionally, due to long term leases held by a few airlines, some airlines at Metro are forced to sublease space from their competitors. New gates will allow these airlines to lease their own gates and avoid paying fees to competing airlines. This gives the subleasing airlines more control over which gates they use and how they use them.