[Glirdan]

This is all, true, but we cannot forget that if he was not slain, Boromir would still be exhausted and need time to rest, so maybe they would have left a little earlier, but not by much to make a big difference in the story's plot. I think they still would have been about a day behind the orcs and the Riddermark would still have killed the orcs at Fangorn's eaves and so on and so forth.