I'm ambivalent/neutral about this idea. If it was done, the intention is clearly for the deletion to happen automatically. An implementation suggestion:
1. The expiry date would be optionally added at page creation or editing. It would not be a default. Removing the expiry date on edit would convert a page to being a normal permanent page.
2. An automatic addition of some text (e.g. an expiring informative template ) added to the affected pages 1 month before their expiry date. This would also show up on recent changes. This gives interested parties a chance to change the page to a permanent page, or copy the content elsewhere.
3. Deletion would occur automatically when the expiry date occurs, (i.e. a month later in this example). This would also appear on recent changes. So the answer is yes, the page vanishes at this point.

This still leaves the child pages issue. This is the most vexious issue, and I don't see a satisfactory answer. Perhaps indicate auto-expiry pages to the user? A different background ground colour & expiry date indicated on the page somewhere. This would apply to the page and all its children ( sub-pages and forum pages ) for auto-expiry pages and their children. {Individual forum pages having an expiry date would be bad. Parts of a thread would disappear, making nonsense of the conversation left behind.} This sub-page auto-deletion would perhaps still be non-obvious to neophytes, even with a different background colour. I don't like the idea of confusing newcomers to SL. Newcomers are important. I don't want them to be bitten by an interface they perceive as complicated. So I still feel I don't have a satisfactory answer to the child pages issue.