Because an aircraft has rear mounted engines, or even an tailengine (like the 727) is does not nesecerly need T-tail, just look at the L1011 Tristar. In each case, you get some bennefits, but must pay at another counter, so to say. Example, you get more rudder for less weight useing fuselage mounted stabilators, but you pay at the drag counter. Useing T-tail give better flow bennefits, but you pay at the stall "counter" Most T-tail aeroplanes have shorter fuselage from wings to stabilator, proberbly because the extra weight of the engines only allows a certan length. If designed with fuselage mounted stabilators, you run into flow problem, because the stabilators are to close to the wash from the wings. So putting them higher solves that problem.
One is not better than the other, it just depends a lot on how the aeroplane is designed.
[This message has been edited by Roger Turbojet (edited 30 December 2000).]