I think the booktable is a growing approach
they cant do door to door so easily now - people are becoming more litigious especially here in australia where lawsuits far outnumber the US
its also a less confrontational approach and it allows them to "proclaim the name of Jehovah" using their big banners etc - which they obviously cant do at doors.
Remember, for witnesses (just as with Hare Krishnas) its all about getting Gods name to the people.