The torsion bars are the rear version of hood springs. They make the trunk lid "pop" open. If you had a fiberglass trunklid you'd leave them out and use a prop rod instead anyway. It's your choice to use them or not. If you don't, you'll quickly realize that you need SOMETHING to keep the decklid open while you remove the spare or whatever.
Incidentally, if you overadjust your torsion bars the decklid can be used as a catapult to throw small objects onto your hood.