1. Because you're worried about the worse case and the space saver spare not inflating when you need it.
2. Because you envision yourself being in situations (trip out of town, off road) that you'll need a 'real' tire if one fails.
3. Because a full size wheel/tire (inflated or flat) will not fit, and can not be secured, in the space under the cargo area.

I've seen what a loose wheel can do to someone in a accident so number three is the one that really rubs me the wrong way. I realize that it's becoming the norm but it's reprehensible for _any_ vehicle manufacturer to put a design on the road that doesn't have a provision to securely store the flat tire once it's removed.

 

Good points. I just don't like the look of the rear mounted spare. I will take my chances with the space saver spare (which is the norm on over 50% of the cars sold). I carry hold down straps in the rear of my Cayenne, so I feel comfortable I could secure a flat in the rear if necessary. All I am trying to say is that the lack of a "real" spare doesn't set the Cayenne apart......the majority of cars sold don't have one. My 997 doesn't have ANY spare, just a can of goop. Now that is cause for real concern.

 

You are right; emergency towtruck/flat repair is not covered if the flat tire is from external forces. However, removing the wheel with the flat from the vehicle and installing the spare tire is covered, as my wife experienced with her '05 CS.

 

My solution was to purchase a used tyre and rim from e bay. Cost me around $200. Yes it's big and takes up room as I have a subwoofer in the trunk. However I feel a lot safer now as the "gloop" and pump supplied is a joke!!!

 

Purchased our first Porsche 3 weeks ago. beautiful black 87k miles. looks great except for the practically new Toyo tires
Just completed passing on a two lane mountain highway and was back down to 60mph when the right front blows out the sidewall. Get off the road, unpack the camping gear to get to the spare. I repeat what has been said earlier, 'know how the spare and air pump work.' practice inflating that ridiculous balloon donut of a tire at home. When I tried to air up the spare it wasn't completely on the rim and wouldn't inflate. With no cell service we had to hitchhike back to town, get a hotel, and wait for Monday and an open tire shop. Took two different tire stores before one of them could seat the bead. (well over 60 psi when it seated!) When I finally saw what the spare looked like inflated I ask if they had a used tire that was close to the correct size because I had a two hour drive back home and no way in hell was that spare going anywhere but back in its hole. $54 for a Yokohama with the wear bars showing and I was happy to pay for it.
Thinking four new Michelins or Bridgestones...

 

Got rear ended in my SL63 last month. A Mercedes rep called in to see if I am ok before I got the car pulled off the road, conferenced me in with 911 dispatcher, cop was there within 15 minutes. BMW had the same system, although I never had to use it. I can't understand why Porsche doesn't have a system like this? When I was buying the vehicle the sales rep told me that they are too small in volume to have something like this. Well, if you combine it with everything Volkswagon owns it would seem that they could do something like GM and their OnStar system.