I think it really depends on several things, including the country you're living in.
Over here in Germany, dealers usually grant testdrives WITHOUT company mostly to customers they know or customers who are part of the local community like doctors, lawyers, people with their own private business/company, etc.
It also depends on the age. Don't expect to get a 996 Turbo or even "only" a 997 Carrera for a testdrive if you're only 18 years old and they don't know you (or the parents).
It also depends on the car you want to testdrive. The more powerful and the more expensive the car is, the lesser the chance to get a testdrive. Most dealers give out Turbos, GT3 only to known customers or people they know they can deal with that power or pay for repairs.
Cars like the GT2 or Turbo S are VERY seldom available for a testdrive, we had to get a GT2 from Porsche directly to testdrive one, this shows how difficult it is.
A Carrera GT testdrive? Forget about it. Even as a potential customer or even if you ordered one, your chances are almost NULL. I had the chance to drive one on an empty parking lot only, it was nice but not satisfying.
The CGT drive in Jochen Albig's (german race car driver and Porsche test pilot) co-driver seat on the track was much more fun, even if I didn't drive it myself.
And a last thing: sales people usually "smell" customers, especially the ones who are older. So if they think that you want a testdrive just for fun, forget about it, they won't give it to you.
You can't really blame them, they want to sell cars and they want to avoid problems with accidents, liability, insurance problems, etc.
A good family friend who owns a BMW dealership just told me about an 18 year old who came to the dealership to testdrive a Mini Cooper S. He had his driver's license for only 2 months. The sales person refused a testdrive but offered to him a testdrive as a co-pilot instead. The 18 year old got very upset, they almost had to call the police.
There is no "right" to get a testdrive, dealers can agree or not. If you can't get one in the driver's seat, accept one as a co-driver. If this isn't satisfying, well, just wait until you really can afford buying one and try again.
I know this sounds harsh but dealers don't sell Porsche cars for fun (well, maybe a little bit...
) but for making profit and making a living. So every person who wants a free ride for fun in a Porsche actually costs them a lot of money and time. Time in which they may have actually sold a car to a "real" customer.