The roadside breath test can be used only when there is a reason to use it. The result is not admissable in court other than to show why they decided to arrest the person and take them in for an admissable breath test. The cops are fully aware of this, and it is not likely that they would risk their career to do random breath tests.
The results of the road side test are not admissable period. The officer has to have other probable cause to make the arrest, the preliminary breath test is for their information.