Because all the infrastructure level things like ACLs (as in this
discussion) won't have to be reinvented. The Sling resource API would
evolve and become more complicated - and loose it's original purpose of a
simple read oriented API, simpler than JCR in comparison. But not a
standard.
Well, but you would be locked in with Jackrabbit, right? So it's not
really plugging in to JCR, but Jackrabbit, if I understand correctly.
Not really. Using the Jackrabbit SPI stack you implement a JCR
repository (the connector) based on infrastructure code provided by the
Jackrabbit project. It doesn't need to have dependencies on
jackrabbit-core however.
Michael