.
Says he who has recieved, but also given, quite a large number of Thank You's:
Those who don't like it can ignore using it, but it is as Sean says, good manners and common courtesy to specifically thank someone who has helped you.
I raise a finger (not my one-finger salute) or a nod to those drivers who stop for me on a pedestrian crossing or who let me out at a junction etc etc and I enjoy it when say a biker acknowledges my road courtesy etc. It spreads positive vibes and goodwill and there's too much discourteous and inconsiderate behaviour in the world nowadays. No wonder some folks go around with long faces.
What goes around comes around.