You raise an interesting point, Evita:

As portrayed in the movie, Roth was surrounded by reporters, cameramen and FBI agents as soon as he landed in Miami, so there was no way for Rocco to kill him except by getting right next to him. And, it seemed, Rocco's only chance to escape was by running--not much of a chance at all.

The point that has been debated here is why Rocco was selected for this seemingly one-way mission. In the preceding boathouse scene, Michael rejects Tom's suggestion that it would be impossible to kill Roth: "If history has taught us anything, it's that you can kill anyone." Then he immediately says, "Rocco?" I've argued (a stretch, I admit) that it's significant that Michael didn't say, "Al?"--thereby selecting Rocco for the one-way mission. Why? For Michael's purposes, Rocco was expendable, Neri wasn't. Rocco was Clemenza's man, Neri was Michael's man. Why did Rocco accept? No alternative especially considering that Michael had just humiliated Tom for saying it was impossible to kill Roth in the Miami airport.