Why? I've turned my account off before in attempt to walk away... Curious why you would set out to do that as an policy. Why not just turn notifications off and share little to avoid cacheing of data?
Why do I do it at all? Being off Facebook is good for my mental health-- this is the main reason. Privacy reasons are secondary but also important. I've posted some really stupid stuff in 2004 that I'm glad is (theoretically!) stricken from the record.