The OEM ecu is fine for a basic swap, stock 1uz with auto or manual transmision. How ever if you plan to go with forced induction, or cams, Itb's or other moderate to extream changes the factory ecu just can't handle them. There is also no way to rechip or flash the stock ecu as far as I know so most people just swap it out for something that's very flexable and easy to work with. I am running my 1UZ swaped supra on a mega squirt 2 for example.