Are these use case the same or similar to just doing it through API access to some set of underlying relational databases?
That question suggests that you see "The Semantic Web" as being exactly equivalent to "HTML with semantic metadata added". But that most definitely is not the case, and "API access" is part of the Semantic Web. An HTTP based protocol for SPARQL has been around for forever, and continues to evolve, as do the standards around semantic discovery of API services.
The whole RDFa, microformats, microdata, GRDDL thing is just a small part of the overall picture.
"There's a group of people I like to call "Semanticists" who've come to latch onto Semantic graph projects, not as a technology, but as a religion. They're kind of like the "6 minute ab" guy in "There's Something About Mary"
...
There's then another group of Semanticists, they recognize the approaches that have been proposed have kind of dead-ended, but they won't publicly announce that. Then when some other approach not affiliated with the SW makes progress (language understanding AI for example) will simply declare this new approach as part of the SW and then claim the SW is making progress."
Some of these things in your list were proposed specifically because the Semantic Web technology inventory had failed to produce anything useful.
Some of these things in your list were proposed specifically because the Semantic Web technology inventory had failed to produce anything useful.
Well, that's one possible narrative one could believe in. Another would be that a lot of people engaging their NIH tendencies invented a parallel technology track, covering a lot of the same ground, and offering no real advantages, just because they could. shrug
That question suggests that you see "The Semantic Web" as being exactly equivalent to "HTML with semantic metadata added". But that most definitely is not the case, and "API access" is part of the Semantic Web. An HTTP based protocol for SPARQL has been around for forever, and continues to evolve, as do the standards around semantic discovery of API services.
The whole RDFa, microformats, microdata, GRDDL thing is just a small part of the overall picture.