... ever heard the legend that goes something like, "nothing so useless as runway behind you"?
As I explain to my student pilots, it might save 30 seconds or so, but just imagine going through the hedge/fence/gate at the other end of the runway by 2 metres, when you could have gained yourself an extra 20m? Or that engine failure after takeoff that you could have got back down onto the runway with stopping distance to spare.... It's not so much to do with takeoff performance as 'stopping' performance. Perhaps when they start fitting ABS to aircraft we won't have to worry about such things..... but until then, I'll continue to encourage my guys to take the extra few seconds.