(Of course, I believe that implanting IUDs in, say, fifteen year olds is disgusting.)
I volunteered for a pro-life crisis pregnancy center a while back. I asked about the statistics of those coming into the center who actually were on some method of birth control and still became pregnant. The answer surprised me. The number one method of birth control being used by those who had conceived was birth control pills. The problem, as you noted, was not the method so much as the consistency factor.
Planned Parenthood knows this as well, and it helps their bottom line. They freely distribute birth control pills to their clientele, knowing that many of them will not take their pills consistently and will return to get abortions.