I do not know the history of it, but in answer to 'is it safe?' - provided all the criteria are met - long and dry runway, rated thrust, no tailwind etc, it is the SAFEST way to go. Think about the best place to be when the engine goes bang. On the ground of course. Improved climb keeps you on the ground longer. Then when airborne, greater margins over everything - Vmca, obstacles etc. Similar argument can be applied to scheduling V1 max for the RUNWAY, regardless of being at a lower weight than max allowable under the conditions. Maybe it results in a bit more tyre wear, but how much does that really cost on takeoff? We did this on the old Viscount, so I suppose it was a crude form of improved climb that dated back to about the time of the B707.