A triathlon bike with tri bars is built to rotate your body forward, giving you aerodynamics without sacrificing the leverage you can put on the pedals.
A road bike with standard tri bars causes problems. It keeps your hips shallow but puts your upper body low, thereby reducing your hip angle and sacrificing power.
The key for those who are riding a road bike is to use “shorty” bars. You can make your own shorty bars by just adding a set of clip-on bars to your road drops and shortening up the extensions.
If you take a look at ITU racers, this is the setup they use. It’s a lot cheaper than getting a tri bike if you already own a road bike. Dan Empfield, of slowtwitch.com, wrote a nice article on shorty bars here.