As others have mentioned, limp mode is not just a transmission control feature. It is a fail safe built into the Engine Control Unit. When all sensors are operating correctly, the car has a map to determine the appropriate air/fuel mixture taking into consideration temperature, pressure, exhaust etc to ensure your car runs optimally. When in limp mode, the ECU cannot trust the sensors to determine the optimal air/fuel ratio. There is a base map that will allow your car to run with a rich fuel mixture (safer than lean) to prevent damage to the motor. Besides being worse for emissions and fuel economy, you can drive the car normally until you can get the issue repaired.