I would love to see rails-api become a first-class player in the Rails ecosystem.
That said, other than increased usage, I don't think being included in core would actually do anything that we can't already do ourselves outside of core.
1. unify people who are looking for a standard approach to create api in Rails
2. clear separation between lower level http support and higher level html/js/css processing
3. performance improvement against using regular Rails for api
4. easier upgrade for app developers