BC does not always work and should NOT be relied upon by teens who are definitely unable to provide for offspring. BC is fine for adults who realize that it might fail and are prepared for that eventuality.
As for abstinence, these unsupervised kids don't even have anyone who cares about them patiently and forcefully telling them why it will be far better for them to keep it in their pants. Instead, the people they most respect (best friends, rap stars) are telling them to do it all night. Abstinence doesn't stand a chance in that environment. And no one cares enough in their families to even make it a POINT to encourage abstinence. How could they? I see these kids at the malls or shopping areas. No parent is in sight.
Of course sexually active people of all ages should use contraception if they are not ready to become parents.
Nothing's perfect, including conntraceptive methods,which is why physicians often recommend using redundant methods. But using contraception gives you a better chance of avoiding pregnancy than using nothing at all. Do you agree?
Abstinence is fine, if you can do it, and should certainly be encouraged. It is good for many reasons,not just avoiding pregnancy and diseae but also for avoiding psychological traumas from premature sexual experimentation and for avoiding heartbreak at a young age.
Unfortunately, not everyone has the strength of character to do remain abstinent. These are people at risk, people for whom contraception is a must, imo. I frankly don't understand why anyone would think that sexually active teens shouldn't rely upon contraception; it's certainly less risky than using nothing at all.
Stats on effectiveness of contraceptives:
http://www.mckinley.uiuc.edu/health-info/sexual/birthcon/thnk-bc.html
Your mileage may vary.