The increase in accuracy from the tracking wheel comes from the lack of wheel slippage. Not an increase in resolution. In some cases, it might make more sense to use the IME than the quad encoder for the odometry data –– for example, if you know the robot will be in a no slipcase (or can ensure it), than you can get higher resolution data from the IMEs and thus be more “accurate”.
Friction isn’t a term I’d use for the cause of slippage. Rather, the motor is providing a torque against the friction from the ground which can lead to slippage if the static friction is overcome. If you manage the torque output/speed of the wheels, you can do odometry accurately and without seperate tracking wheels. There’s actually a way to detect wheel slippage and factor it into the position tracking but you’d need to individually power each wheel.