Parry is only worth it for one-on-one duels in light or no armor. Even then you need a pretty hefty skill score at it to be effective. If you are making a themed PC who is something like a professional fencer or duelist, its worth considering for the RP potential. It, plus improved parry for the counter attacks, improved critical for luck, and expertise / improved expertise to swap into when you are being slaughtered. Thats a hefty chunk of effort just to compete against one foe.
Against the overwhelming majority of monsters, don't even bother. It is indeed, worse than useless. The traditional monster "mob" will not respect your fancy swordwork and happilly bite / chew / maim you from the sides as you focus on your single opponent.
Ironically though, parry is actually kind of handy to have for a monk. Not as much as some things, but a monk nonetheless has a good setup for it.