At least for Talk, Google gets the latest sitemap files daily which makes the results more current. Along with sitemaps, Google will usually get new posts in the forum within an hour.

Yes, deleted threads can be a problem, as with deleted or moved pages in the wiki. For the one searching, if he/she gets a 404 error (page not found) most probably he/she will change his search terms, as what I usually do when searching via Google.

I think the missing pages problem is quite small compared to the advantage we will get on being able to search the whole site in one page.