For most people contraception is more about when to have kids as opposed to how many. Once you've had your 2.1 kids most people will find some permanent method (e.g. vasectomy or tubal ligation). But having an insurer pay for your pills/IUD/etc just means that they're paying to help you time your childbearing. So I can readily see where the cost savings would be very slim.
Besides, this goes back to the perversion of insurance. Insurance has historically been a means to spread risk of an unlikely but catastrophically expensive event occurring. E.g. you don't expect to wreck your car but replacing a $30k item is more than you can afford so you pay $50/month to State Farm so that if you do wreck your car you get a check for $30k and they also write checks to anyone whose property you damaged or who were injured. Now insurance is more like a pre-paid maintenance program in addition to the traditional insurance. Having health insurance pay for contraception (and for that matter normal childbirth) is about like having your car insurance pay for oil changes and tire replacement.
