Iron X is a time saver.... using an iron remover or tar solvent after washing, but before claying will save you a LOT of time when it comes to the actual claying process.... that is assuming the vehicle suffers from iron contamination and/or tar contamination to start with. If the vehicle does not have any tar build up or iron buildup, then clearly it makes no sense to use these types of products. Simply do a spray of IX on each panel or so to see if you get a reaction, then if needed, go around and thoroughly cover the entire vehicle and allow it to do it's job.
Time = Money as a professional, therefore the added expense of using IX should be negated by the added time savings. A gallon of IX is $100 retail (if you're a pro and are paying retail, you aren't doing something right), you may use 8 oz of IX, and therefore 16+ vehicles per gallon so $6/car retail at most for a product that should save you 30-60 minutes when it comes to claying if the car is contaminated with iron particles.
Work smarter, not harder
-Zach