Shows that, as a form of 'muscle', they were hardly a threat to the Corleones.
They were a treat to Pentangeli, not to Michael. At least not directly. And I'm not even sure if the Rosatos saw Michael as an enemy.
Btw, the Rosatos are closely modeled on the Gallo brothers, including the scene when Pentangeli is nearly killed. The whole Rosato-Pentangeli conflict is based on the Gallo-Profaci conflict.
"It was between the brothers Kay -- I had nothing to do with it."
