For me, Will Smith has not defended Jada - he has instead twisted the conversation to one about violence ands about him. Now, all we are talking about is how he hit Chris Rock and whether the joke justified the smack.
Without the slap, we have Chris Rock making a cruel joke - perhaps even a bullying one. Called out, the talk would have been about how wrong Chris was to target Jada like that. By hitting him, suddenly the talk changes tangent to whether or not Rock deserved the hit.
He could have did all he did, minus the slap (i.e. go up to Chris, tell him to 'keep his wife's name out of his fucking mouth' etc) and it would have still been a strong statement about how wrong Chris was. Suddenly instead of a straight forward condemnation of the joke, we have this nuance about whether or not Rock is both the bully of Jada and a victim of Will. That's a real shame (for Jada).
(Out of all of this, Denzel comes out looking like the coolest of the cool.)