My thinking on this is that a 6.0L that has a current calibration installed, is running properly, the engine oil is not overfilled and isn't using crap fuel, EGR failures due to carbon are simply not an issue any more. Unlike in the past, we see very few EGR problems that are not affected by an EGR cooler failures, poor performance and poor maintenance practices. I have attached TSB 08-26-12 for us to review... though many of us have determined the baffle is pretty much wothless the description and diagnostics are important.
As maintenance I also think it is a waste of your customers' money and your time. If you have a build up of carbon and you are not addressing the cause of it you are not doing any good in the long run. But I have been in this business long enough to know that our employers are out to make money and offering these services are one way of doing so. If your customers see any benefit despite not being long lasting, then I guess all is not lost.
When repairing an engine that has had coolant or oil contamination in the intake you are better off having the manifold hot tanked as this will also clean out the passage between the cooler and the valve which the "flush" simply can't clean.