A B737-800 FO COMPLAINS OF THE ATC PROCS OF APCH CTLR IN PLANNING TFC CTL AND REDUCED SEPARATION BTWN ACFT WITHIN THE CLASS B AIRSPACE OF PHL; PA.

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I WAS THE COPLT AND PNF ON ACFT X; A B737-800; APCHING PHL FOR LNDG. WE WERE AT 6000 FT MSL; ON A 190 DEG HDG FOR PHL; 10 NM N OF THE ARPT. TFC WAS CALLED BY ATC AS A NAVAJO DSNDING TO 6500 FT; 4-5 NM ON A NORTHERLY HDG. WHILE MONITORING THE TCASII; WE RECEIVED A TA; WHICH WAS TO BE EXPECTED. I CONTINUED TO MONITOR THE TCASII TO ENSURE THAT THE TFC LEVELED AT HIS ASSIGNED 6500 FT. THE TFC WAS NOT IN VISUAL CONTACT. APPROX 3-4 SECONDS AFTER THE TA; THE TCASII SYS RESPONDED WITH AN RA. THIS TIME I NOTICED THE TFC ON THE TCASII WAS 3-4 MI AT +400 FT ABOVE WITH A TREND VECTOR SHOWING DOWN. THE AUTOPLT WAS DISCONNECTED AND A DSCNT WAS INITIATED TO 5000 FT IN RESPONSE TO THE RA. ATC WAS NOTIFIED IMMEDIATELY UPON BEGINNING OUR DSCNT. THE LOWEST ALT THAT WAS REACHED WAS APPROX 5500 FT. UPON THE TCASII SYS ANNUNCIATING 'CLR OF CONFLICT;' WE RETURNED TO OUR PREVIOUSLY ASSIGNED ALT. IT WAS REASSURING TO KNOW THAT THE TCASII SYS DOES IN FACT WORK FOR BOTH TA'S AND RA'S. I DO FEEL THAT UNLESS BOTH ACFT HAVE EACH OTHER IN SIGHT; THAT TFC SHOULD NOT BE CTLED SO AS TO CAUSE RA'S ON THE TCASII SYS. AS PLTS; WE ARE TAUGHT TO RESPOND IMMEDIATELY TO CERTAIN ADVISORIES (GPWS; WINDSHEAR; TCASII; ETC). TO EXPECT US TO IGNORE CERTAIN ALERTS IN AN EFFORT TO INCREASE TFC CAPACITY DOES A GREAT DISSERVICE TO OUR TRAINING.