Google might not register it, but in some related testing, even a few hundred requests through the API (serial, not concurrent) takes much longer than a single "Past" page request would, even if the latter were substantially rate-limited. Of course, if those requests (to HN itself rather than the API) are blocked entirely, that's a moot issue.
On dead/flagged items, there's some value. Whilst the title/URL context aren't available, just knowing what fraction of submissions and comments are moderated is interesting data, and it is possible to construct patterns against specific accounts.
I'm frequently encountering what appear to be banned accounts. Being able to trace those through the API to see where and when they were banned, or now much moderated activity they're generating, can be useful. I'm relying heavily on the "/replies?id=<UID>&by=<moderator>" search endpoint (generally dang, tomhow, sctb, or pg as mod) currently to find out if there was a specific ban admonishment from a moderator. That's often but not always the case.
But it's not possible, say, to tell through the API what sites are banned. Looking at site history with "showdead" enabled can tell you that though, e.g.:
I've not worked with the API, and there's the blessing/curse (blurse‽) that HTML is a known, if poor, standard.
API always translates to "one more thing to learn, that's applicable to a single-use case". HTML scraping / sorting I can apply across multiple sites.
That said, a standard, say, JSON packaging of website contents available on request might be fun to have.