IT is a question of having takeoff thrust set at the commencement of the take off roll to ensure the the performance requirements can be achieved.
Boeing state that the increase in takeoff distance is "negligable"
However how do you quantify what is negligable?
I have seen many guys/gals roll many many metres whilst they stuff around like old women setting 40% N1 and then dithering a bit more for good measure then hitting the TOGA buttons.
Rolling starts, whilst the norm, have the "potential" to invalidate the takeoff performance figures. Before people tell me that I am alarmist, I think you will find that the takeoff performance figures are predicated on having takeoff thrust set within the line up allowance - which for most smaller jets is within the piano keys at the end of the runway.
John Tullamarine can expand upon my comments more eloquently than I can.