Thank you? How many times is enough?

I read a blog about thank yous and it got me thinking. How many times do you have to say THANKS? I mean come on if you open a present in front of someone, you say you love it, THANK YOU SO MUCH, why is it considered rude if you don't send a follow up thank you card? Does the giver really need to be stroked that much? Another example: You go to lunch with someone, they pay, you say thanks, do you have to send a follow up? I have received a follow up. Okay how about this: You have a kids b-day party, they tell the other kid thanks for the present. Then how about this idea: The parent of the kid who gave the present send the mom of the kid whose birthday party it was a card saying "Thanks for spending a butt load of money on my kid and taking them off my hands for the afternoon. It was really great having the afternoon to myself". See where I am going with this? Instead, the stressed out mom who is now in the poor house from spending a butt load of money on a party now has to worry about thank you cards. And that means basically writing them out for your kid who is screaming at the table saying" I don't want to do this it's too hard " as they are looking at a stack of 20 cards to fill out. Do you blame them? I am all for a nice Thank You card but sometimes I think a verbal and a smile is just as good.