ACR CAPT DESCRIBED FMS LIGHTING PROB THAT RESULTED IN FMS DEFERRAL; ATC WAS CONFUSED BECAUSE OF THE NEW ICAO ACFT SUFFIX INFO.

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THERE I WAS GETTING READY FOR AN EARLY DEP; WHEN I NOTICED THAT THE FMS BACKLIGHTING WAS INOP. I COULD SEE THE KEYPAD AS THE DAY BEGAN TO BRIGHTEN AND THERE IS A FLOOD-LIGHT ABOVE THE CONSOLE; TOO. BUT I ELECTED TO WRITE UP THE ITEM EXPECTING MAINT TO DEFER THE COCKPIT LIGHTING UNDER MEL SECTION 33. I WAS ASTOUNDED WHEN MAINT DEFERRED THE WHOLE FMS; NOT THE RELATED COCKPIT LIGHTING. THIS NONSENSICAL DEFERRAL SERVED TO DECREASE SAFETY OF THE FLT AND CREATED A GOOD DEAL OF EXTRA WORK FOR THE CREW AND ATC. (I MADE PRETTY POINTED PROTESTS TO MAINT AND DISPATCH ON THIS MATTER.) ISSUES: 1) WAS MAINT CORRECT? IF SO; HOW ABOUT SEPARATE MEL RELIEF FOR FMS BACKLIGHTING; EITHER IN SECTION 33 OR SECTION 34? 2) CLRNC DELIVERY CTLRS IN BOTH STATIONS STATED THAT THEY WERE STILL CONFUSED BY THE ICAO FLT FORMAT. THE FIRST CLEARANCE DELIVERY GUY SAID 'WE DON'T SHOW YOU RNAV ANYWAY.' DESPITE THE 'G' IN THE EQUIP LINE AND THE STANDARD 'RNV????' FLT PLAN REMARK; CTR CTLRS KEPT CLRING US DIRECT-TO FIXES; TOO. 3) THE RETURN FLT PLAN SHOWED THE FMS DEFERRAL AND DROPPED THE 'G' BUT STILL HAD THE RNAV REMARK AS ABOVE. (THIS REMARK WAS DELETED BUT THE SUBSEQUENT FLT ON THIS PLANE; THOUGH.) THE NEXT TIME I SEE FMS LIGHTING INOP; I WILL CERTAINLY BE TEMPTED TO IGNORE THE PROB; RATHER THAN HAVE NO FMS.