My reading of the question "Why has a proposal for Zend Framework been closed?" is that if there's a community demand for the site, then it's okay for a sub-topic to become it's own site.
What's not clear to me is how a "vote-to-close" based on a topic being a sub-topic of an existing SE is a duplicate of a site? Seems to me that's not a reason to close, and that there should only be pointers to existing tags within SE sites that might already cover the topic as needed.
More to the point, I don't agree that closing a proposal based on it being a sub-topic should ever be a reason to close a proposal. Meaning if there's enough people interested, then there's enough people interested, and that demand in my opinion means that SE would benefit from the new site.