I agree with you on motivation. Weight Watchers uses a similar system, they just make you measure weight periodically in front of a person of opposite sex. If you are serious about weight loss, and obviously you are, physical exercise has only 20% impact, diet is the real thing.

I loose weight before sessions too, and I think everybody does. You want to look your best. The thing is life is complicated, losing weight is feasible, keeping it off very difficult, and if you mix work with play sometimes you get to conflicting goals for both parties. On one hand you are serious about it, on the other it becomes an opportunity to play.