I think the concept of 'you writing a letter' to dissassociate yourself is something that the WTS has spread itself for two reasons:
1. Like ISP said, if they announce that someone has dissassociated themself (even if they havn't) and everyone believes that they must have writtena letter then it gives them a way to disfellowship people even if they have not done anything wrong.
2. If they
can get people to write a letter it does afford them some measure of legal protection I would have thought.
I would not recommend writing a letter even though I felt like doing it at the time. It really doesn't bother them and means you are sill playing by their rules.
Better to tell as many people as you can about what you have learned and try and save the friends you have (although you may find very few were 'real' friends, just assosciates in the same club).