Well I think the reason they make you re-check luggage is more than just clogging the train. What if I flew in with my knife collection in my checked suitcases are they going to let me wander out into the concourse assuming that I will make my way out of the secure area. Security does not want to have to screen everyones checked luggage it would take forever. So they send it out and you pick it up outside the secure area.

Also you think that LHR and other Airports are secure but SEA may think otherwise insisting that you get rescreened in order to enter the secure part of the airport. Who knows what airport you flew in from and how secure that foreign airport is. I certainly question LHR security as I have had things stolen in the secure area by B.A. or Airport employees and the airport claims to have no cameras recording people's faces to track down thefts in the Airport.