The trouble is that if you do not know if "it is broken" if you never check and clean the res. Virtually all chemical nutrients are going to give you some salt buildups, making cleaning of the res necessary. In addition, plants do not uptake nutes evenly. When you do not empty your res and just add it it, you really have no idea what is in the nutrient solution that is left in the bottom of the res. I also think you invite problems if you do not clean out your res periodically.