First of all, there are already plenty of established userland
implementations - so there is really no need for this.
On the contrary, plenty of implementations means there's a need in this
functionality, and it might be a good idea to have one standard
implementation if it can cover like 80% of use cases.
I agree with Stas on this one; I have noticed that there is a real need to
parse the meta-data from a DocBlock (not just tags, descriptions as well).
phpDocumentor is an example but other projects benefit from this as well.