When isn't an IRI enough of a value in JSON-LD? - json-ld

Question

I'm playing around with JSON-LD but I think I'm missing something about linking. Gregg Kellogg's answer to Repeat whole JSON-LD structure or only the ID? almost clears it up for me, but not quite.
He has an example like this, where the author is an organization:
{
...
"author": {
"#id" : "http://example.com",
"name": "My Organization"
}
}
Which he simplifies at the end to look like this:
{
...
"author": {"#id" : "http://example.com"}
}
It seems to me that if I wanted to link to something, I should be able to simply reference it:
{
...
"author": "http://example.com"
}
That value would be seen as an IRI (assuming author is defined correctly) and that something would inflate the object at that IRI. However, almost all of the examples I see have extra stuff around that. What am I missing here? What can't I do if I just use the IRI instead of the method Gregg uses? He says that some people repeat parts of the graph, but that would seem to be a huge problem in managing data should I have to change a name. Is there a practical or theoretical problem there?
The sample on json-ld.org seems to be what I expect, but the things I've seen most often don't do it like this (linking to spouse):
{
"#context": "http://json-ld.org/contexts/person.jsonld",
"#id": "http://dbpedia.org/resource/John_Lennon",
"name": "John Lennon",
"born": "1940-10-09",
"spouse": "http://dbpedia.org/resource/Cynthia_Lennon"
}
I think in the end if doesn't matter because Markus Lanthaler also notes that you can't rely on parsers to inflate objects so you're stuck specifying everything all the time just in case.