AN A300 CARGO FLT HAS A POTENTIAL CONFLICT WITH A BONANZA XING HIS DEP RTE OUT OF DTW ON AN ASSIGNED HDG CLBING OUT OF 2000 FT. TFC WAS NOT CALLED; WITH THE SMA ON TWR FREQ AND THE A300 ON DEP CTL. RPTRS WERE CONCERNED WITH ATC'S METHOD OF TFC HANDLING.

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WE HAD JUST DEPARTED DTW FROM RWY 3C; WERE HANDED OFF TO DEP CTL; AND ASSIGNED A HDG OF 050 DEGS. ON THIS HDG; CLBING THROUGH APPROX 1500-2000 FT MSL; WE SAW A LIGHT SINGLE ENG AIRPLANE (IDENTED BY THE CAPT AS A BEECH BONANZA) AT OUR 12 O'CLOCK; PASSING R TO L. WE ESTIMATE HIS VERT SEPARATION AS 500 FT AT THAT MOMENT (WE WERE CLBING TO 10000 FT MSL ON THE PUBLISHED GENEVA 2 SID) AND DISTANCE 1500-2500 FT. WE PASSED BEHIND THE SMALL PLANE WITHOUT TAKING EVASIVE ACTION. CAPT ASKED DEP CTL ABOUT THE CRAFT. WE WERE TOLD THAT THE TWR WAS 'TALKING TO HIM' (OR HE WAS 'TALKING TO THE TWR') AND THAT IT WAS AT 2500 FT MSL. WE DID NOT QUESTION THE DEP CTLR ANY FURTHER. EVIDENTLY; THE SMALL PLANE WAS TRANSITING THE TWR'S AIRSPACE VFR. THAT HE WAS TRANSITING ACROSS THE DEP PATH IS WHAT DISTURBS ME. DTW WAS USING 2 PARALLEL RWYS FOR DEP; WITH FLTS TAKING OFF ONE AFTER THE OTHER AT A VERY BUSY TIME OF MORNING. HAD OUR RATE OF CLB; SPD; ETC; BEEN SLIGHTLY DIFFERENT; A COLLISION WOULD HAVE BEEN VERY LIKELY. MY OPINION: DURING A BUSY TIME OF DEPS; NO VFR ACFT SHOULD BE ALLOWED TO TRANSIT SUCH AIRSPACE; REGARDLESS OF WX CONDITIONS; PARTICULARLY ACROSS DEP PATHS. THE TWR PERSONNEL HAVE NO WAY OF KNOWING WHAT OUR RATE OF CLB MAY BE; AND THUS ARE NOT ABLE TO PREDICT OUR CLB PATH WITH ANY PRECISION. NOTE THAT THE GENEVA 2 SID INCLUDES A RESTR TO BE AT LEAST 5000 FT MSL AT 10 MI; AND MOST PLTS WILL CLB SO AS TO LEAVE NO DOUBT ABOUT MAKING THE RESTR. SUPPLEMENTAL INFO FROM ACN 381812: HAD WE BEEN MAKING A MAX PWR TKOF; WE WOULD HAVE BEEN EVEN CLOSER. NO EVASIVE ACTION WAS TAKEN. MY CONCERN IS THAT THE TWR CTLRS UNDERESTIMATE THE CLB CAPABILITIES OF THE NEW; LONG RANGE TWINS AT LIGHT WTS.