I am not trying to put down the product at all - it honestly looks like it does an excellent job and for 95+% of people out there this is more than good enough for their needs. And yes, I imagine that if a car has plastic wheel well liners you could theoretically clean them out with a product like this, provided that you've got enough clearance around the wheel to get in there with something. But there's no way you're getting the salt off of the cars underbody; floor pans, suspension components, etc no matter how many brushes and MF towels you've got.
For me, that's my main concern in the winter...but like I said, the majority of people just want the car to look clean and shiney and probably don't give a second thought to stuff like that.