RPTR EXPRESSED CONCERN ABOUT THE TWR'S SEPARATION BTWN HIS ARRIVING ACFT X AND DEPARTING ACFT Y.

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WE (ACFT X) WERE CLRED TO LAND WITH A VISUAL APCH AT 4000 FT MSL; FOLLOWING A CITATION JET (ACFT Z) 4 MI AHEAD. AFTER CITATION Z TOUCHED DOWN; CMH TWR CLRED B727 (ACFT Y) INTO POS FOR IMMEDIATE TKOF. CITATION Z TOOK TIME GETTING OFF RWY AND B727 Y USED ALL 10000 FT GETTING OFF. WE (ACFT X) TOUCHED DOWN AS HE (ACFT Y) LEFT THE END OF RWY. B727 Y SHOULD HAVE NEVER BEEN CLRED INTO POS AFTER WE RECEIVED LNDG CLRNC AS TWR DID NOT KNOW WHAT CITATION Z'S INTENTIONS WERE; NEVER ASKED HIM TO EXPEDITE OFF OF RWY. WHY WERE WE LEFT WITH A LNDG CLRNC WHEN IT SHOULD HAVE BEEN CANCELED? WE TRULY NEED SOME GUIDELINES FOR ATC AND PLTS TO FOLLOW IN THIS SIT. LNDG CLRNCS SHOULD NEVER BE GIVEN UNLESS THE RWY IS CLR. CANADA USES CONTINUE WHEN SUCH SITS ARISE. WHEN I RECEIVE A LNDG CLRNC; I ASSUME JUST THAT -- I AM CLRED TO LAND; AS THE RWY WILL BE CLR. THE CITATION Z WAS A FACTOR; BUT WAS NEVER TOLD TO EXPEDITE UNTIL HE WAS CLRING. THE B727 Y SHOULD HAVE BEEN READY TO GO BUT WAS NEVER TOLD TO BE. AS SUCH; HIS SPOOL TIME ON ENGS WAS LONGER THAN NORMAL. WE SHOULD HAVE HAD OUR LNDG CLRNC VOIDED BUT NEVER DID. I THOUGHT ABOUT GOING AROUND BUT DID NOT KNOW THE HDG THE B727 Y WAS GOING TO FLY; OR IF THE PARALLEL RWY HAD TFC ON IT TAKING OFF. LNDG SEEMED THE ONLY OPTION AVAILABLE AS THE TWR DID NOT SEEM TO BE AWARE OF ALL THE PLAYERS.