" 你叫什么名字" what is your name? - direct. " 怎么称呼" how are you called? - indirect Direct or indirect, either way, it deserves the same meaning. There is no such theory that speaking in a straight forward manner is impolite.

it;s not rude,but depends,if you ask for the elder's name ,you'd better say怎么称呼您 for kids or people at the same age with you,you can just say你叫什么名字

it depends on the situations, for example, when a group of children are playing together and a new guy is coming, then they may introduce each other by asking "你叫什么名?", it sounds casual here; one more situation, when a police is interrogating a suspect, he can also ask "你叫什么名?", which sounds very formal and iron-handed. "怎么称呼" is used, however, on occasions of first time meeting between adults or business circumstances. As an adult, you should always use the latter to people unless you are speaking with a child, only in that case, you can use the former to shorten the distance between you and the child. Hope this can do for you.