I like WTW. The reason is 'psychological factors.' Those being the network affect and critical mass.
Someone can come out with an identical, or even superior weight loss program, but it will be difficult to compete with the meetings that Weight Watchers holds. They have the critical mass needed for this type of service (similar to Dale Carnegie for anyone that's had the pleasure to take that fantastic course).
The network affect, and I'm no expert on this topic, helps Weight Watchers because it means that anyone looking for a weight loss service, that comes with counseling, etc. has the logical option of using WW. Other programs may be available, but WW has a solid program with group meetings that are robust with attendees (enough attendants to be effective). Once someone feels like they're "part of the group" they're going to stick around WW for a while. They also get the feeling that the other attendees are dependent upon her, so there's an altruistic element here. Attendees don't want to quit because "the other members count on me".
My apologies for those thoughts being somewhat scattered.