Great! But what's the point? So he can guard the 3 but does he have to? He's a guard so he will be guarding the 1 and 2 positions. Does it really matter if he can guard the 3?
Yes... it does. If you can't see the value of a player that on any given possession can switch assignment (because of a screen, pick, etc.), then not worth discussing further.
Not really. The 3 rarely sets picks, the 4 or 5 does most times. I'm pretty sure Smart can't guard 4s or 5s. And if by any chance the 3 sets the pick, the bigger guy is now guarding Smart's man (quicker guard) and they'll just take advantage of that match-up.
Switches do not happen on every pick by the way; teams actually hedge and rotate back to their match-ups most of the time. So again, Smart guarding the 3 doesn't have some HUGE impact that will severely interrupt opponents' possessions. Pretty sure Stevens would rather have Green rotate back to guard the 3 than to leave Smart on Green's guy, whether Smart can guard the guy or not.