Stainless is more important on unleaded petrol fueled vehicles than diesel. In an unleaded, the sulphur dioxide (rotten egg gas) in the exhaust mixes with condensed water (before the exhaust heats up enough to evaporate it off) and forms sulphuric acid. At least that is my understanding. It's the reason why exhausts on unleaded petrol fueled vehicles "rot" from the inside-out, and why so many modern petrol fueled vehicles have stainless steel exhausts.
Not so much of a problem with diesel - the chemistry is different. But they can (and will) rust from the outside, given enough years. The other thing though, is that on a 4wd that is used for 4wding, the exhaust will often cop enough punishment over the years to need replacement from damage before rusting through.
