It might be difficult for admins who are unaware of a subject to also block users from pages related to the pages they are blocking. This will allow people who are blocked to edit similar pages to what they are blocked from.
The WMF's Anti-Harassment Tools team is going to finish up existing functionality of Page and Namespace blocks and will release them to several Wikipedias before deciding if we'll start on any new features. We want to see what needs come from real-world usage of partial blocks before committing to anything else.
The concept of this ticket may still be valid and useful, but I closed because there is slim information documented so far.