Porsche did receive a patent for the under-tray design, so there is an apparent aerodynamic benefit according to their engineers.
If it's worth anything, I keep it on my cars (daily driver C4 and race car C4).
Check out:
http://forums.rennlist.com/rennforums/forumdisplay.php?s=&forumid=59
...do a search there and you'll find all the information you could ever want to decide on your own
