Well if they do it right, a smartphone app based detector could be better than most current machines, because of app flexibility. It should also be considerably cheaper than "whole" models.
I imagine being able to choose between accurate simulations of popular current model detectors, and maybe even some vintage models. It would be like having a CTX3030, an Etrac, an Equinox, an ATPro, ATMax, And various Fisher, Whites, etc. models, all rolled into one! Also, original coils from these machines could be used, maybe some with adapters. The simulator machine should come with a main "universal" coil, and have an assortment of coils available for purchase.
That's if they do it right...I think. My imagination may or may not be realistic.