So, yes, if someone wants to include the value of happiness when evaluating a given purchase, that ROI logic can be deemed legitimate (the logic itself, that is, the logic as it is consistent with itself).
Using that logic, let's say that for me personally, a clean, shiny truck would provide more happiness than a truck with a TrailerTail installed. So if we assume that a $7.00 bottle of truck wax and one-day's labor spent waxing the truck produces 10 units of happiness over a one-year period (the useful life of the wax job), and a TrailerTail on our truck would produce five units of happiness, then the wax job would have a vastly superior ROI to the TruckTail because (1) the cost of the wax job is much lower, and (2) the units of happiness are greater.
Now, since the TrailerTail is installed only once and the wax job must be done once a year, it could be argued that the total TrailerTail happiness would come to exceed the wax-job happiness over time. But, since non-financial reasons can be included in an ROI calculation, and since I really want to be happier with a wax job than with a TrailerTail, I can achieve my happiness goal by preserving the ROI logic but changing the happiness input to give the wax job 100 units of happiness instead of a meager 10. After all, on a sunny day, the freshly-washed truck would make me really, really happy!
Many purchases are prompted and justified by non-financial reasons and there is nothing wrong with that. If being happy with your purchases matters most, then the amount of happiness a given purchase provides also matters most.
But for someone who is serious about increasing one's profit in a trucking business, other considerations command greater attention and require deeper and more objective thought.
So, if someone wants to complete a true financial analysis of a straight-truck TrailerTail purchase, we need the numbers. For starters, we need to know, how much does it cost to install a TrailerTail on a straight truck?