This was experimented with in BA on the 737 LHR fleet about 10 years. Very limited usefulness. You had to try and get airborne at roughly the right time. We all know how successive approaches and a take-off queue can seriously mess that up. Then really there is a fairly limited range of speed available to the crew to control Fix Arrival time. So large variations in cruise/descent spped, combined with manpower planning and issuing RTAs- why not just let aeroplanes get away when they can and ATC controls them inflight and handles them during descent as needed than plan for flights that may well not make RTAs? It's something that looks great in planning that doesn't stand the test of the real world.