I sometimes pull from rack becaouse it lets me handle weights far heavier than i can pull from the floor, and it is not as taxing as pulls from the floor. If it helps the lockout? Yes, in a way. Doing rack lockout or speedpulls from the floor and "**** the bar" (hipdrive) so to speak is good for getting an explosive lockout, atleast it helped me some. What really made my deadlift fly was when i started to do cleans, and alot of em.
On a side note - heavy rackpulls build great trapzorz, which attract girls, which is win =)