Please don't back me up by talking about doing Goodmornings on a Smith Rack. I can fight that fight by myself if need be.
It does crack me up that the same guys who rip the Smith are often the guys lunge walking the length of their gym with dumbells in hand.
I've lunge-walked, lunged pushing back, done step-up lunges. I've lunged with dumbells, lunged with barbells and I reassert; no lunge beats a lunge on the Smith. It hits your gluts better and for athletics you can perform it more ballistically.
Also, when I had a pec injury I was able to bench in the Smith where I couldn't with a free bar. It was good for rehab. I'm not a Smith lover or anything but I recognize it as a tool that has some use.