Can't think of many advantages of the dash mount ignition, except perhaps that it simplifies the wiring loom and the key hole is easier to see. Also - custom and tradition, important to many drivers.
Advantage of the steering column position - mainly that it facilitates steering wheel lock on key removal. Disadvantage - knee injury in certain impacts.
Centre console position - facilitates gear selector lock on key removal, arguably as effective as steering lock. Considering how much work is necessary to get at the locking mechanism, I would think its more secure than a steering lock. Disadvantage - dirt might get in the key slot.
Saab's official reason for the floor mount ignition was to reduce knee injuries in accidents. Apparently people's knees made contact with keys. I'm not sure I believe that, and one also must ask why Saab chose the floor and not the dash as Mercedes and Porsche did... even as Saab originally had! But apparently they believed something.
The shifter interlock was a response to the anti-theft requirements introduced in the '60s and '70s. I think the Germans lead the charge around 1960 and the US fell into place around 1970. Most cars chose the steering wheel lock, but since the 99 had a floor mounted ignition switch the shifter got the lock.
My understanding of the 9000 was that because it was built on a common platform (with the Thema and the 164) and Saab was the only one doing floor-mount ignitions that it had to be sacrificed. The story goes that after the prototypes were built and Saab was completely dissatisfied with the results and re-engineered all sorts of things but it was too late to redo the wiring harness. Probably if they'd known the shared platform wasn't gonna be that shared the ignition would have gone down in the console.
I don't think any lock is really that much of a theft deterrent. A screwdriver and a hammer will defeat most of them.
Fair to assume Some 'Marketing Moron' convinced the factory that a ' typically ' mounted key would please/entice North American buyers.
Just like the laughable 'Erectile' radio antenna was a Sales feature..