Actually I can. If I saw Dwayne Johnson in a SWTOR movie at Jace Malcolm I wouldn't think: "Hey look here's comes Jace Malcolm!" I'd be thinking: "What's the Rock doing dressed up as a trooper? This April Fool's or something?" While he may be able to perform the role adequately (though not exactly well) - and his voice would be miles behind that of Kevin Micheal Richardson - he would not be suited for the role. Johnson lacks any real kind of versatility unfortunately.
You can't tell me that if they were casting a SWTOR movie today that Dwayne Johnson would not be an absolutely perfect fit for Jace Malcom. He could ace that same sort of role (if there was one to be had) in the new trilogy.
EDIT: Don't take this as a bash at Johnson (although it kinda is ) but I don't think he's good enough to really add anything to the franchise.