I can see appeal of these 'lift bars' getting the car higher with regular jack stands, and if you do a lot of your own work. I am sure they also give the frame some nice added rigidity.
But as stated above, the unibody design does not really need it. There is a crease of metal that runs from the front to the back wheel wells which can support the car, and you can jack the front and the back from this point to place jack stands.
Source:
Jacking the Boxster
Porscha
Threaded Mode
