Both ways will work - one better than the other. The plates spread the heat to the general floor area by contact. The tube alone spreads the heat by relying on convection air to move the heat to the rest of the floor. The output of tubing without the flat plate is generally lower, sometimes half that of the tubing with the flat plate.
Adding the flat plate makes the job a more definite success. If your heating needs are small with a lot of floor area, installing the tubing without the flat plate may be enough, but why take a chance? You might have to add some baseboard later to make up for inadequate heat from the floor.