The big censorship debate

Having just found out about the BBFC banning The Human Centipede Part 2 (interesting article here http://www.guardian.co.uk/film/filmblog/2011/jun/07/human-centipede-sequel-ban-sexual-sadism?CMP=twt_gu ) it got me thinking about censorship today. I was actually rather surprised to hear of the BBFC's decision as it feels that in this day and age anything goes. With films such as Irreversible (2002) depicting graphic rape scenes and many more explicit and violent scenes in gore porn films like the Saw franchise surviving the final cut we are not used to films being 'banned'.
The 1984 Video Recordings Act meant that Straw Dogs (1971) was banned by the BBFC due to the violent rape of Amy, and more notoriety and controversy was to follow in 1999 when an edited version of the film was again refused certification by the BBFC as it believed there was "the clear indication that Amy comes to enjoy being raped". In 2002 an unedited Straw Dogs was finally certified for DVD as the inclusion of the second rape scene, according to the BBFC, showed Amy did not enjoy the act of violation. Incidentally, Hollywood has remade Straw Dogs and it is to be released later this year. I am not going to discuss this remake in terms of 'why remake it?' as yes I do not feel it will offer anything new or improved on the original but that is digressing from my point. Instead, I am interested in how they have dealt with that inevitable rape scene and if Amy will 'enjoy' it. Will there be censorship issues for this Hollywood version?
So just how much can film get away with today? It seems there remains certain limits, particularly surrounding sexual perversity, but do we need censorship? If we don't want to watch a horror film with horrific acts in then we don't have to, we can make our own decisions can't we? With everybody now discussing the banning of The Human Centipede Full Sequence (2011) there will be a mass of people wanting to see it and no doubt going out of their way to see it illegally, so what has this censorship really achieved other than a great marketing ploy for the film?