EMB120 PLT RECEIVES VECTORS OFF AIRWAY WITH NO STATED REASON FROM ZLA.

Narrative

THIS HAS BEEN A RECURRING EVENT AND HAS THE POTENTIAL TO CREATE A CONFUSING OR UNSAFE SITUATION. WE WERE IN CRUISE ON A VICTOR AIRWAY WHILE TALKING TO ZLA. CTR TOLD US TO 'TURN R HDG 315 DEGS.' I READ BACK THE HDG AND ASKED FOR A PURPOSE FOR THE VECTOR. ATC DID NOT REPLY. I REPEATED THE QUESTION. ATC SIMPLY REPEATED THE INSTRUCTION TO FLY HDG 315 DEGS AND AGAIN DID NOT GIVE A REASON. THE CAPT OVERHEARD THE EXCHANGE AND ALSO ASKED ATC THE PURPOSE OF THE VECTOR. HE EXPLAINED THAT WE NEEDED TO KNOW FOR LOST COM PURPOSES. ATC FINALLY RESPONDED CURTLY WITH 'TFC.' SEVERAL MINS LATER; ATC QUESTIONED OUR INTENT IF NO REASON FOR THE VECTOR HAD BEEN GIVEN. WE RESPONDED THAT WE WOULD NOT HAVE MADE THE TURN BECAUSE THE CLRNC WOULD HAVE BEEN INCOMPLETE AND CONFUSING IN THE EVENT OF LOST COM. NO FURTHER EXCHANGES WERE MADE REGARDING THE MATTER. THIS FORM OF PLT INTIMIDATION IS POTENTIALLY HAZARDOUS SHOULD AN ACTUAL LOST COM SITUATION OCCUR. AS PLTS; WE NEED TO KNOW WHAT TO DO IF THE RADIOS FAIL; AND IT IS ATC'S JOB TO INCLUDE THE PURPOSE OF THE VECTOR WHEN VECTORS ARE INITIATED. AT THE VERY LEAST; ATC ABSOLUTELY NEEDS TO RESPOND WHEN ASKED ABOUT THE REASON FOR A VECTOR. IN OUR CASE; ATC DID NOT GIVE A REASON UNTIL BEING ASKED 3 TIMES.