First off, WTF is up with disabling copying? That is seriously annoying. Now on to what I came here to say..

"When there are enough of these tombstones, the hash map gets rebuilt, but that’s OK. Due to the magic of amortized constant time, this doesn’t slow you down on average."

If amortized time is all you care about, that's fine. But there are some applications where worst case time is more important, and unfortunately I've seen some hash map implementations that implicitly assume that the worst case never matters.