What if I have two dozen ruby-related tags, not just rails? In order to search all of them without hierarchical tags, I would have to either add the "ruby" tag to all of them, or run a search with a lot of ANDs.

I already do this to some extent with my 11,000+ bookmarks on Pinboard. Entering the Usenet-style tags is really easy thanks to autocomplete, which also helps me avoid ending up with misspelled tags. Pinboard currently lacks searching by tag prefix, but it's trivial to script it locally using the API.