I personally use the Smith for a lot of stuff that I have a hard time holding my lower back on (existing lower back problems) - shrugs, SLDLs and bent over BB rows.
However -- yes I've seen people do the SMith leg press. I've done it myself once w/ my trainer last year. My question is, how the fuck do you get the bar OFF the rack w/ your feet? You can't. You have to have someone roll it off for you. THEN, how the fuck do you get it locked back IN?? You have to roll the bar. And you're layign down there w/ your whole torso exposed to this friggen bar that could fall down on you because it rolled off your shoe???
Screw that buddy, I'll stick w/ the leg press.