I'm with you on liking to see unusual cars- though I can see the potential problem if a combination of modern developments and suitably lax eligibility standards were to allow something with no real competition background in period to be 'hot-rodded' into a highly competitive car.
Otherwise, I'm happy to see them- even at international level, there's usually been someone campaigning an unlikely car in most categories of motorsport (eg the Porsche 928 you mentioned- I'm not aware of the 928 having any rally history, but French privateer Raymond Boutinaud raced one a couple of times at Le Mans and a few other international sportscar races around 1983/4:
http://www.amicale928.org/media/Lm83c.jpg ) so I don't have any objection to that being reflected in historic racing