They're handy for checking for fault codes, and for getting readouts of the sensors inputs (i.e. CAS, AFM, TPS voltages etc), plus for viewing/logging timing adjustments wrt RPM and time (combined with a wideband O2 sensor and its software, and using consult, I managed to log RPM/timing/AFM/AFR/inj duty cycle all on one graph).
If you're tuning via stock ECU remaps then I'd DEFINATELY recommend one. I can see the advantage of the serial ones with the LCD screen you can get if you're on the edge of your injector or AFMs capabilities and want to monitor it, otherwise you can get plain serial ones for quite cheap or USB ones (they have a serial->USB adaptor built in).
If you're not gonna keep the stock ECU, well not much point really obviously.