In my, admittedly, somewhat limited experience that is not true. I used to work at a minilab and processed quite a bit of XP2 for both myself and the customers who had primarily bought it from the lab where I worked. I also did a lot of cross-processing of E6 in the C41 machine, and that didn't contaminate anything either. I think a bigger issue of chemical lifetime is properly maintaining and cleaning the machine and at times, stirring the chemical troughs, at least that was part of the maintenance routine I ran on our machine every week.