Of course TT is what matters, but the Coach still play a vital role. Lets take an example of 12649/50 itself. It is an ICF rake train, capable of running at a max speed of 110 kmph. Now, SC - KZJ section is capable of handling trains at 120 kmph, further north, BPL - NZM is capable of handling 130 kmph (BPL Shatabdi hits 150 kmph and Some RAJ's hit 130 kmph - 140 kmph), but due to rake capacity, KSK will not be crossing 110 kmph mark. So if this train was given LHB, it could run at at least 120 kmph, hence reducing travel time. 22685/86 is a slow train, hence 12649/50 deserved that LHB Rake!