Wow, I don't think I'd recommend your travel company to your friends. It certainly sounds like they have a pretty poor grasp on the US.
Just for some clarification, The "I" in a highway number stands for Interstate, the 4+ lane superslab freeways that criss cross the US. There actually is no Interstate 50. There is a US-50, which we've been talking about. US highways are generally speaking secondary routes to the Interstates. They often are 2 lane highways, and are slower because they go through cities and towns. That said, they can also be ideal routes for roadtrips because they allow you to get a much more realistic feel of america than what you often see just off the interstate.
If I were you, I'd find a few things that you'd consider "must sees" and then build your route around it. For example, between the boneyard and Tombstone (among many other great places), it sounds like Southern Arizona would have lots of things you'd be interested in seeing, but they would be difficult to do if you limited yourself to following just one highway.