Keeping PPM in check does not mean you are keeping the amount of different ions that make up that PPM in check. For example, If your plant for some reason uses all the phosphorus you add and only half the amount of nitrogen, it's just a matter of time before your nitrogen levels reach toxic levels.
Also, roots break off and die, bennies die, some external microorganisms always manage to reproduce, and with time you get more and more organic waste acumulating in your res (ok, you might not, bennies could be fully breaking down everything but I wouldn't bet on it).
Changing res means starting fresh, both in a context of nutrient levels and potentialy problematic organic matter acumulation. I don't doubt you could go without it for a long time, but I think it's a good practice that can prevent a lot of problems along the way..