ATR42 CREW WAS DISPATCHED WITH AN MEL ITEM THAT REQUIRED A LNDG ON A DRY RWY AT MIA.

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ARRIVED AT TPA OPS AND WAS INFORMED THAT FLT WAS DELAYED. CALLED DISPATCHER AND WAS TOLD THAT BECAUSE ECU (ELECTRICAL CTL UNIT) FAILURE RPTED AND MEL'ED BY PREVIOUS CREW; ACFT WAS RESTR TO 'DAY; VFR' ONLY. ADDITIONALLY; I WAS INFORMED THAT THE ACFT WAS RESTR TO DRY RWYS ONLY. AS IT WAS RAINING IN MIA; THE DEST ARPT; AND BECAUSE IT WOULD NOT BE DAYLIGHT UNTIL XA12; THE FLT WAS DELAYED UNTIL THAT TIME. AT XA45; I CHKED THE WX IN MIA AND WAS ABLE TO ASCERTAIN THAT IT WAS NO LONGER RAINING. PAX WERE BOARDED AND WE DEPARTED AT XA12; AS PLANNED. SHORTLY AFTER TKOF; I CALLED THE DISPATCHER VIA RADIO AND RPTED OUR TIME AIRBORNE. THE DISPATCHER INFORMED ME THAT THE MIA STATION WAS STILL RPTING THE MIA RWYS AS BEING WET. I THEN CONTACTED TWR AND THE TWR CTLR RPTED THE RWYS AS BEING DRY. THE FLT CONTINUED TO MIA AND LANDED ON DRY RWYS; WITHOUT FURTHER INCIDENT. I BELIEVE THAT WE WERE IN COMPLIANCE WITH THE PROVISIONS OF THE MEL. HOWEVER; I REMAIN CONFUSED AS TO WHO ACTUALLY CAN; OR SHOULD HAVE; DETERMINED THAT THE RWY AT MIA WAS DRY. WHILE OUR COMPANY DOES PROVIDE 'FIELD CONDITION' RPTS; THEY ARE FREQUENTLY INVALID. THIS ESPECIALLY HOLDS TRUE AT STATIONS SUCH AS MIA WHERE THE FIELD CONDITIONS CAN CHANGE MIN BY MIN. ADDITIONALLY; WHILE THE WX RPTING IS MADE BY CERTIFIED WX OBSERVERS; THE FIELD CONDITIONS RPTS ARE NOT. AS AN ADDED NOTE; SHORTLY AFTER WE LANDED; I RECEIVED THE COMPANY FIELD CONDITION RPT SHOWING THE RWYS AS DRY. HRS LATER; AFTER IT HAD BEEN RAINING FOR QUITE SOME TIME; THE FIELD CONDITION RPT SHOWED THE RWYS AS BEING DRY WHEN THEY WERE CLRLY WET. IN THE FUTURE; I RECOMMEND THAT THE FLT NOT BE DISPATCHED UNTIL THE DISPATCHER CAN MAKE THE DETERMINATION THAT THE PROVISIONS OF THE MEL CAN BE COMPLIED WITH. THE PLT; OTHER THAN LOOKING AT THE CURRENT WX CONDITIONS; HAS A DIFFICULT OR IMPOSSIBLE TIME MAKING THESE DETERMINATIONS WITH THE INFO AVAILABLE IN THE FIELD.