Some say you get a bunch more hop utilization. Other's say it's not much. My brew software shows a big change in IBU's when I switch to a late extract addition. Yet I don't usually adjust for it and I don't see that much of a difference.
I think it has to do with measured IBU's and what you taste as bitterness. *shrug* experiment with it and see what YOU think and do what works for YOU.
I do partial mash and like to boil with just what I get from my grains, then I add my extract with 15 minutes left or less. Sometimes I add it at flameout.