I see your point, but perhaps some of these students would befriend each other and get sorted into different houses...thus creating more inter-house friendships...maybe even between Gryffindor and Slytherin. There will always be those who will know which house they belong to due to family trends and gravitate towards others who already know as well (children of death eaters for instance).

It would be better to delay sorting, but not to do away with it entirely.