There are other reasons to go on the pill other than to stop pregnancy.
As for the effectiveness
Combined Oral Contraceptive Pill - claim 99.8%
Birth Control Questions - claim 98%
The Pill - Birth Control Pills - Oral Contraception at www.Birth-Control-Comparison.info from Cedar River Clinics - claim 92-99.7%
As I mentioned before using a condom + the pill is pretty much 100% effective.