Actually, Emily Post says (and has for many, many years) that Thank You notes are required for all gifts except those received, opened and thanked in person. She does go on to say that older people sometimes expect a note even then, and it is polite to send one when you know the giver would expect it.
The purpose of manners is being courteous to others. The purposes of thank you notes are to 1) tell the giver that you have received their gift, and 2) to express your appreciation for the giver's thoughtfulness.
The purpose of manners is being courteous to others. The purposes of thank you notes are to 1) tell the giver that you have received their gift, and 2) to express your appreciation for the giver's thoughtfulness.