I always found it interesting that in the moment of crisis for the whole Corleone family - it still was vital to find out where Luca was...shows his street value

keep calling him in the war chamber/inner circle - keep calling all day.

Luca really was as tough as Santino could ever dream of being, Luca scared everybody but the Don - if he had flipped, Santino and Tom and the capos (Tessio, and that fat Clemenza) were about to throw in the towel...