The metal in that location is does not have a straight top edge. Any flat spacer will crush the metal edge in the centre. That's why the Saab tool has a shape designed to match the shape of the bodywork.
When the edge crushes, paint and underseal are lost which won't help guard against the dreaded driveshaft tunnel rot.
The alternator bushings live a very "warm" life. Unless they are constructed out of different material to account for the engine/alternator heat they must absorb, I'd say that the suspension bushings might be expected to last a bit longer.
That said, 20 years of use has prolly extracted most of the usefulness out of them......:cheesy: