Murph, I think the difference between your truck fleet, my truck fleet and someone buying a tractor is that you and I buy vehicles every year that we have experience with. A long distance tractor purchase may not have that luxury of experience with the same/similar model.
If I buy a new GMC or International, I spec it out with a specific Allison transmission that I already own and have experience with. I spec out the engines we already have in our fleet. I'd guess that you do the same. So it is pretty easy to buy a truck, and even a car for the fleet that is sight unseen. We are essentially buying "known" quantitites. And even if it is a new model vehicle, we are buying something that we will have serviced by a local source, so again, if the new model to our fleets is not quite the same, at least the service guys are nearby.
When buying a tractor, unless it is dramatically different from most other tractors (for example a Power Trac articulated tractor) then there is a lot to be said about sitting in and testing the tractor, as well as reasonably local service and support. . . that can vary by region of the country. What is considered "local" in one area may be a very long drive in another area!