I'm not a big fan of hierarchical categories. As already someone said here - often one category in one hierarchy path can be part of another path. I'm also -1 on having that in core, exactly of this reason.
I am a big fan of multiple tagging. You can have many tag fields - tuple fields using a KeywordIndex.
Then again, If you project's hard requirements are having a hierarchical taxonomy, go for collective.taxonomy.
IIRC @petschki used it recently and @jensens has much experience with semantic web technologies together with Plone, where taxonomies play a big role.
We're using collective.taxonomy quite often because it's a simple solution for user editable vocabularies like it was back in the days of ATVocabularyManager. The benefit of the hierarchical approach can be useful, but this depends a lot on your project specifications.
In combination with collective.collectionfilter you get super flexible categorizable and filterable content on your Plone site very quickly. (note for me: it might be worth a chapter in the docs how to setup a groupByCriteria adapter for the collective.taxonomy utilities)
In general im -1 on getting this into core for the same reasons @thet and @fredvd mentioned.
I also think it would be good to have collective.taxonomy up to date and a first class citizen in add-on space. So, do not bloat the core anymore. In fact it would be good to reduce and simplify core features were possible. The beast is difficult to tame anyway with all it's features
