IFR SMT inbound to HOU with I90 experienced conflict at 3000 with unreported traffic; reporter questioned ATC's altitude assignment below Class B.

Narrative

On an IFR flight from RBD to HOU; in VMC; being controlled by Houston TRACON. Arrived via the Blubell 2 STAR. Shortly after passing COWZZ Intersection at 5;000 FT; was given a descent to 3;000 FT and a 135 degree heading; vectors for the HOU ILS Runway 30L. I did not realize it at the time; but this descent took us out of but underneath Class B airspace. Shortly after leveling at 3;000 FT; cruising at about 200 KTS; TCAS indicated traffic at 12 o'clock position; 3 NM; 500 FT below and climbing. I began deviating left (east) but another target appeared about 1/4 miles east of the original target; limiting my ability to maneuver. About that time I acquired visual contact with the original target. It was in a climbing; steep-bank turn toward me. I passed a few hundred FT above and less than 1/4 miles east of it. I never acquired the second target and my TCAS unit indicated that I passed even closer; but west of it. I wonder if it may have been a phantom 'double' of the first target. As I was passing by the traffic; I got a radio call from TRACON suggesting a right turn (I think an earlier call from TRACON may have been blocked). I told them that I had deviated left and was returning to my original 135 degree heading. 'That was close;' I said. 'They are out of the Class B and I am not talking to them;' was the response. The frequency was too busy to ask the question that came to mind next: 'Why am I out of the Class B?' To vector relatively high performance aircraft on approach to a major airport out of Class B is bad procedure in my opinion. No higher performance aircraft were in the vicinity. My understanding is that the purpose of Class B airspace is to avoid exactly this type of hazardous situation. It should be used for its intended purpose.