ACR PIC STATED CONFUSION RE USE OF INOP ACFT EQUIP.

Narrative

ON PREFLT ACFT HAD A FLT CTL COMPUTER THAT WAS INOP. MAINT REPLACED THE FAULTY COMPUTER. DURING MAINT TEST OF ACFT'S AUTOLAND CAPABILITIES THE MECH COULDN'T GET A PROPER INDICATION FROM THE AUTOTHRUST SYS AND AS A RESULT DECIDED TO PLACARD THE AUTOTHRUST SYS INOP AND DISPATCH THE ACFT UNDER MY COMPANY'S MEL. ON TKOF THE AUTOTHRUST ACTIVATED AUTOMATICALLY AS WAS NORMAL. I DECIDED TO LEAVE THE AUTOTHRUST SYS ON AND IT FUNCTIONED NORMALLY THROUGHOUT THE ENTIRE FLT. LNDG AT DEST WAS VISUAL. I HAD CLOSELY WATCHED THE MECH DURING ALL HIS TESTING OF THE SYS AND FELT THAT INASMUCH AS WE NEVER GOT ANY SELF- DETECTED FAULTS OF THE SYS DURING THE TESTING THAT THE AUTOTHRUST SYS WOULD FUNCTION NORMALLY WHEN USED IN 'NORMAL' OP. THE MECH AGREED. WE ALSO ENDED UP FLYING THE ACFT WITH NO PAX AND NO REVENUE CARGO ABOARD SINCE OUR FLT HAD CANCELED. THE PROB HERE IS THAT OUR MEL; WHILE IT DEFINITELY ADVISES US WHEN TO NOT USE A SYS; DOESN'T TELL US WHEN WE MAY USE AN INOP SYS. RECOMMENDATION: ALL MEL'S SHOULD BE VERY CLEAR WHEN AN INOP SYSTEM MAY BE USED VS WHEN IT MAY NOT.