It all depend upon the condition of the mounting holes. I prefer not to use the plastic upper bulkhead since I already have the aluminum ones. The bulkhead itself will not be stressed but there is a little added leverage on the screw holes which is why the spacer works. It adds a little more leverage from the rear spoiler/wing thus enhancing the down force.
On my car it feels very solid. it's much better than just adding a couple of standoffs which don't have as much contact area as my spacer.

Either way, were only talking about $15 bucks.