I forget we have separate subs.An SSBN isn't going to be assigned to protect an aircraft carrier. That would be a "pants on head retarted" - to use an offensive phrase - decision by an Admiral unless said Admiral had no other submarine available.
Sure, a ballistic missile submarine can perform the role of attack submarine in an emergency because they do carry torpedoes. You would have to be an absolute goddamned fucking idiot to assign one to an attack submarine's mission because that's not what it's designed for and the crew probably isn't trained for it.
My later point still stands though. Being able to be stealthy and fast is beneficial, cause once caught, leaving is important