My 2004 GT 3.2 has a full size 17" spare. It very slightly raises one side of the mat which is not a problem. I would hate to shred a tyre or wreck a rim on some of our isolated country roads on interstate routes, where you can be miles from the nearest civilisation and out of mobile phone contact, in a car with no spare. A compressor and goo are no use in those conditions. With a full size spare you can be safely on your way in a short time. Our 159 ti Sportwagon has a full size 19" factory spare wheel, that fits the well perfectly. I think the manufacturers leave out spare wheels in order to reduce weight and gain a tiny bit of fuel economy to meet regulations. I reckon it should be illegal to have no spare.
2004 GT 3.2 V6, Grigio Stromboli
2008 159 ti 2.4 JTDM Sportwagon, Grigio Stromboli
1989 75 3 litre V6, black
1988 75 3 litre, red, (son's car now)
2000 156 TS 2 litre, Cosmos Blue, (daughter's car)