Agreed that this is a big step, but what is being included in core is very distinct from Prismatic Schema. For example the way keysets vs values are handled is intentionally different from schema.
Yes, I've read and see the difference from schema.
The huge advantage with this, is that even though the approach is different, it supersedes schema's abilities and it will be a require away for every library and application author.
The rationale is a good read.