necessary...???? i honestly dont know; its necessary to have a hero and villain in any good vs evil story. but theres alot about sw that is just grey area. vital? yes. it was important to show a least one jedi who walked to a different beat AND was still one of the good guys. someone who followed the rules not because he wanted to but becuase he had to set an example. this is yet another way for us to understand obiwan better, since quigon is his mentor. alot of what we see obiwan do in 2,3 & 4 is the result of being seasoned and not new to everything like luke was. it would be out of character for obiwan to discover anakin and present him to the council. only the last request from quigon compelled obiwan to take anakin for apprentice. he trained luke out of guilt over what happened in the past, out of a need to make things right.