It most certainly is a method of contraception. Like all the others, it has advantages (always available, nil financial cost, etc) and disadvantages (needs trust, doesn't protect against STIs as much as condoms, etc).
Because of the 'it isn't really a method of contraception' stuff, it's often not included in surveys and studies, but have a look at the Oxford Study, when it comes out as about reliable as condoms for long term users. Have a read of Germaine Greer's Sex and Destiny: The Politics of Human Fertility for some of the back story.