Gaddzooks! I'm now an 'evil author that publishes books on vehicles...'! Is that a select club I have joined unwittingly by .....
p**lishing?? 
Just joshing with ya, fellars...... I don't see much of a link between us 'evil authors' and displaying vehicles outside, but I have to say that even I have been guilty, in a museum environment, of offering up a 'sacrificial tank' for outside display. But this was on the basis that the museum had two more historically significant examples tucked away in storage. It wasn't a particulalry rare type, but the exercise did appease the ex-armoured corps persons who were giving the Director heaps about not having a publicly displayed tank in the grounds. So add another factor to the museum decision 'mix': external pressure groups, of which this is but one example of many.
As for outside display, I have to say that objects are estimated to deteriorate at least 15 times faster outdoors with no cover, than indoors away from sun, snow, wind and rain. Even a simple roof cuts down the rate of decay significantly.
So for objects that were, in good faith, placed outside many years ago, surely it is a matter of priority: work to move or place those at most risk (which includes its degree of rarity) under some form of cover. Of course, easy to say that sitting in front of the computer theorising, but for most museum collections, not an unreasonable aim or expense.
Mike C