The mechanic probably read the owners manual, page 226 FUEL REQUIREMENTS, "If you buy good quality fuel and follow the cold weather advice above, fuel conditioners should not be required in your vehicle". This is for a 2003 Dodge/Cummins.
I, however, do not agree (not that I'm an expert).
In '03, these trucks & engines were brand new with little over the road experience on the part of Dodge or Cummins. From listening to seminars at May Madness, given by the "Head Man" at Cummins, I know that they are still learning, upgrading & adjusting their thinking regarding these engines.
At another seminar given by the Las Vegas area Bosch Rep. , he said that it was a very good idea to use an additive that added lubricity, to your fuel. Especially, if you are getting fuel from refineries producing ULSD. As an example, CA is producing ULSD. If you are living in a surrounding state that gets fuel from CA, you are, most likely, getting ULSD & not know it. Bosch has, apparently, seen a pattern of damage caused by the use of "low lubricity" diesel fuels & has warned their repair facilities of the problem.
In my personal experience, I've had an ongoing problem with an injector knocking, for a year, now. DC would not fix it because it did not show "as a problem" according to "there" testing. Out of frustration, & after listening to the Bosch Rep. , I tried Primrose Power Master 405. It took awhile but, the problem is gone. Others here have had similar experiences with other products, also.
It seems that if you don't run the "crap" out of the engine, carbon can build-up on & in the injectors causing them not to function properly. Some of the additives seem to eliminate the carbon build-up. I have now started running Marvel Mystery Oil on top of the Primrose 405 for additional protection. That may be overkill but, I don't want anymore injector problems & a bit of extra lubricity in the fuel should help preserve the HPCR & injectors.
In my opinion, I believe running an additive is a wise move.
Joe F. (Buffalo)