DC9 CONCERNED WITH EMER STATUS OBSERVING DFW CLR SF340 FOR TKOF WHEN ON A 3 MI FINAL.

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WE HAD PERFORMED A PRECAUTIONARY ENG SHUTDOWN DUE TO AN ENG START VALVE OPEN ANNUNCIATOR LIGHT (VALID INDICATION). WE DECLARED AN EMER AND THE NATURE OF THE PROB TO DFW DEP AND BEGAN A RETURN TO DFW. SHORTLY AFTER BEING HANDED OFF TO REGIONAL TWR; LCL CLRED A COMMUTER TURBOPROP FOR TKOF ON RWY 17R; THE RWY WE'RE CLRED TO LAND ON. THE SF340 LIFTED OFF AS WE WERE PASSING 1000 FT AND WE LANDED WITHOUT FURTHER DIFFICULTY. MY CONCERN LIES IN THE FACT THAT IF THE SF340 HAD HAD TO ABORT ITS TKOF AND EVAC ON THE RWY; WE WOULD HAVE BEEN FORCED TO ATTEMPT A SINGLE ENG GAR AT MAX LNDG WT; MINUS 400 LBS -- AN UNNECESSARILY RISKY MANEUVER. IT SEEMS AFTER CHKING THAT ATC IS UNDER NO OBLIGATION TO INCREASE SEPARATION AHEAD OF EMER OPS. IN OUR CASE; THE SEVERELY DEGRADED GAR CAPABILITY IMPLIED BY OUR ENG-OUT CONDITION; MADE INCREASED LEAD SPACING LOGICAL AND VERY DESIRABLE. I RECOMMEND THAT ATC BE COMPELLED TO INCREASE SEPARATION AHEAD OF EMER ACFT WITH DEGRADED GAR CAPABILITY; WHICH IMPLIES THAT PLTS EXPLICITLY DECLARE SUCH DEGRADED GAR PERFORMANCE WHEN DECLARING THE EMER.