The point is that the upper bound model is much, much easier to run, since you don't have to do any complicated modeling of how much of the substance people are actually exposed to, you just assume the worst case. If this model already gives you the right answer, it saves you the considerable work of coming up with a realistic exposure estimate (and then the additional work of explaining this estimate in a way that people believe you, etc.).

So it's not a case of running 1 model vs. running 2 models, it's running 1 dead simple model vs. 1 much more difficult and complicated model. If the simple model is good enough, you just want to run that. In this case it wasn't, though.