No if the heater comes on the defrost thermostat is OK as it is in series with the heater.
One proviso is that the defrost thermostat can go intermittent, often you can see that it is bulged out.
Keep an eye on the defrost timer it could be that it is hanging up at one spot in it's cycle.
Only way to tell is if the unit does not do a defrost which sounds like your original problem. I would probably replace the timer and perhaps the defrost thermostat just to be safe. The heaters rarely go intermittent.