Smart is valuable on defense because he generates more steals, turnovers, deflections, crazy hustle blocks etc than other defenders. He's also very good at ball denial and otherwise blowing up actions as called as well.
The best defenders often don't wow you by stopping a guy, they erase potential good positions from the opposing offense. If you don't know what the other team is trying to run its hard to properly evaluate if this even happens.
Anyways if you think he's bad against quick PGs, I guess everyone in the NBA is bad against them? Most of the time he's getting "beat" by quick guards its on the high screen and roll I assume? Given how the NBA allows such screens to be set, that's not really on Marcus himself the whole team has to defend that action properly. (The big man has to help, the wings have to dig down, and everyone has to recover)
He definitely hasn't been as good since coming back from all his injuries on defense, but man one game where a guard has a good shooting night and suddenly 5 years of work in the NBA is discounted....