I was thinking more of owners of 3 or 4 year old cars with 3000 miles on the clock
Sorry, Michael, I didn't realised you're refering to Simon
I quiet agree with your statement regarding 3/4 yr old cars with minimal mileage, my thought are that certain components could deteriorate more or even seized such as calipers, piston rings, water pump, bearings etc etc. due to infrequent use, I could be wrong...
Cearealsurfer could probably able to answer the downfall of such low mileage car(if there's any
Not driving a car enough can cause all sorts of problems... however things have become a lot better with more modern materials and bearing designs.
However in terms of pure storage by far the biggest effect is the quality of storage solution. The average garage is not fantastic and dry-proofing or using a car-coon can increase the longevity of corosion caused by the atmosphere.
The second largest form of corosion I usually see on morgans is UV. A lot of the rubbers are on the exterior of the car (& TYRES!!) and these can quicly loose memory, sag and crack. Trad perspex windows can suffer as well... becoming crazed around the edges. Certain colors do not survive well either.
As for internal corrosion, fuel lines, brake lines/calipers can all deteriorate if not driven OR regularly monitored and changed at service intervals. For longer storage terms draiing the systems and properly moth-balling the car can conserve a considerably larger amount of the original parts. Older oil lubricated seals can perish if stood for longer periods... a tell tale sign is oil leaking out causing small puddles. These seals will often quicly perish if driven upon and will continually leak only getting worse.
The WORST cause of car deterioration is washing a car and putting in the garage. even if the bodywork is dry the mechanicals just start to rust and deteriorate. I personally tend to wash the car before use rather than after to allow the car to dry on the move using the heat of the engine/exhaust/air friction and sunlight to dry it off, returning the car to the garage only once it's dry.
A long term stood car on the surface will often look pretty clean and tidy. On closer inspection they tend to have lots of smaller niggly problems that often can cause a LOT to fix/resolve.