Mormon gives us this information to contrast the slaughter of a thousand Lamanites (with certainly a large number of wounded) with the lack of any Nephite death and only fifty wounded. This great disparity in casualties shows the superiority of the Nephite position, something that would have been quite impressive to Mormon, a military man in his own right.
What is interesting is that it is noted that the unprotected legs were the locations of the injuries. This is not only an accurate depiction of the vulnerable points in hand to hand combat, but it also strongly suggests that these wounded were from the fighting at the entrance, since that appears to have been the only point at which such hand to hand fighting had taken place. Those on the walls would have had their legs protected by the earthen wall, or perhaps wooden stakes in the top of the wall, which were added for further protection.