I think that Michael deliberately chose to hit Roth in such a public setting to further establish his reputation as a ruthless,invincible Don who would let no challenge go unavenged.
True,he could have done it in a quieter fashion,distancing himself from potential incrimination, but I believe that the whole point was to prove to those in his world that no matter what,no matter where,no matter when, he would get you if you crossed him.
Even if Rocco lived, I don't think he would have flipped,and without that,there really wasn't anything for the cops to hang their hat on.
As with Fredo,Michael had to show that no one could get away with crossing him.
Proof or no proof,the most important thing to Mike was that everybody in the Mob world knew that he killed Fredo, Roth,the heads of the Families, Sollozzo,and McCluskey.