For a couple of good reasons.

1) Make use of unused space...right where VW put the spare tire.
2) Get more space in my luggage rack (where I had been carrying my
spare tire.

As to putting a rack on the back? As someone's recent photos
showed....if you or some other car has a tire that extends out beyond
the bumper, you or the person that hits you are likely going to end up
with a much more expensive repair bill.

I'd much rather have to replace a bumper, than screw around with a
damaged corner and rear hatch.

10psi? What did you think I was carrying the air pump for? I'm not
going to be able to fix a flat in 5 minutes, even if the tire was fully
inflated. You grab the spare, start the air pump on it bring it up to
pressure, get the flat off the vehicle. By the time you are ready for
the spare it will be up to pressure. And I have to carry the air pump
anyway for off road adventures, as I lower the air pressure in my tires
if I'm going to be offroad for an extended amount of time.

As to the flimsy remark. It's made of 1/2 inch metal rod. It is more
than enough to carry a spare tire. The spare tire carrier is NOT
supposed to act as a skid plate (there was something completely
separate for that, and my Tristar has it.) The spare tire carrier on
Vanagons is actually pretty thin metal.