I can't speak for the manufacturer but ...
The integral temperature sensor is a relatively new addition to the K Series of controls and it makes perfect sense that they would offer this feature on the control which offers digital temperature adjustability. While the K30 does allow for temperature adjustment, this is done by removing the face plate and adjusting a set screw. Not exactly user friendly and far less precise than the K60. This is a logical step in encouraging the purchase and use of the recommended control.
I would recommend upgrading to the KT60
as opposed to adding the sensor. The control size is exactly the same so no drilling is needed and the difference in your steam shower experience will be immeasurable!