Your phrase:
> largely ignored and we're doing just fine without it
is a pretty good description of how it's going here. Let be be a bit more specific. We used to build lots of SOAP services, and maintaining the schemas that clearly specified the interface was a real effort. When we moved to APIs that used JSON, at first we did little in the way of documentation - and as usual, doing no documentation worked fine at first.
After the JSON API use grew to on the order of 300 developers in teams across the country "no documentation" didn't scale so well. Instead, we've been using swagger specifications to document our APIs. First of all, the swagger specification is far less detailed and precise than the XML schema were. Secondly, my observation (not carefully measured) is that only about 80-90% of the swagger specifications are even accurate. And no one cares. Apparently the level that works for us is just slightly above "no documentation" but well below "clear and accurate specifications".
And I'm (mostly) OK with that.