The thermistor is used during the carging process.It detects a high temperature in the battery pack and interupts (stops) the carging process until the temperature returns to normal.The charger for my cordless drill uses the same thing.
The fuse is in series with the battery,and would open the circuit in the event of an "overload".I don't want a fuse in my receiver power supply.Even in an overload condition,I would rather risk destroying my receiver and servos.And have at least some chance of getting the plane on the ground,without property damage,or personal injury(to myself ,or others.
This is just my opinion! You are free to make your own choice,and I to mine!I personaly choose to remain in control and attempt to fly the plane to the site of any eventual "Crash"!