Not completely true. The Pap smear is primarily a screening test for cervical cancer and associated HPV, but hormonal changes (which can be associated with ovarian conditions)can also be seen, as well as other infections like Candida which are not necessarily sexualy transmitted. Although rare, metastatic disease can sometimes be picked up as well. In my experience it is pretty accurate - I think they quote 95% - but there are the ones that slip through. It is only a screening test rather than a diagnostic test, which is why it is very important to keep having them during your life. Although it is not 100% effective it is better than nothing.
It doesn't take very long, and is worth the brief 'uncomfortableness'.