I think that's only part of what makes Smart great, though it may count for more of what makes Smart a fan favorite.
Smart has great anticipation, especially on defense (but the story of his offensive imprvement in recent years reflect that as well). It's the fact that he recognizes developing plays earlier than most players that makes his his hustle and aggression useful. He's aggressive with a purpose because he knows what is coming.
He's also worked to make himself a versatile defender. He plays PG, and he's fairly big for a PG, so he's going to be matched up against smaller, quicker players a fair amount of the time, and he can do that. But he also understands how to match up against a lot of wings and even bigs, where he is the smaller quicker players. The Celtics "switch everything" defensive schemes depend on Marcus being able to hold his own agaisnt bigger players.
Finally, on the Celtics he'll often be the coach on the floor directing the defense (usually this is a center because they tend to have the whole play in front of them), but also the guy who will spot a bad matchup and do a scram switch - on off the ball switch intended to get a defender out of a bad matchup). Or sometimes, he'll see the offensive team setting up a pick and roll designed to get him off the ball - and he'll execute a switch with whoever is defending the pick setter so that the pick actually switches Smart onto the ball.
He's a really brillent defender in multiple different ways - focus only on his recklessness misses some of his best stuff.
