Note: I am not implying or arguing that a user should ever be restricted from deleting their post - this question is about acceptable etiquette on the part of other users after a post has been deleted.
Occasionally, a user will delete a post after it has garnered some answers. As such, the post has become useful to some degree, in that research which may be helpful to other users has been done. In the case of a meta post, useful conversation may have occurred.
One such example is Should questions about sex and other adult activities be off-topic? (Sorry, it's deleted, so it requires enough rep to see it now.). The question generated 10 answers, and a lot of discussion over our policies regarding "adult material". The OP decided to delete their question and their account. But the subject of adult material has come up again from time-to-time. It would be useful to be able to refer back to the original conversation, but we can't since it's been deleted.
Another example is this recent story identification question.
The questions were asked, research was done, Sean Duggan's answer was incorrect, but the OP found the correct answer. Instead of writing up a new answer of his own, the OP left a quick comment and then deleted the whole question.
As Sean Duggan pointed out in chat, though, doing so removes useful knowledge from the public site. We have no idea why the OP deleted the question in this instance, and we don't really need to - it's their right to delete it if they choose to.
But technically anyone with enough rep can vote to undelete a post. When is it acceptable (or unacceptable) to do so? Undeleting posts is not something we see with any sort of frequency here - in fact, I can't think of a single time I've ever seen it done.