Selected answers from the Dumb SEO Questions G+ community.
Alan Bleiweiss: In my experience, across dozens of audits a year, for several years, category silos done properly, help a lot.
Steve Gerencser: And done right, it also helps immensely with site navigation and User Experience for users.
Michael Martinez: There is no reliable testing method to prove or disprove this belief. "Done right" about says it all for virtually every strategy that people have argued and fought over. A little bit of luck helps, too. If you`re the first into the niche you could do some things wrong and still come out on top for a long time before someone displaces you. The search algorithms don`t really care about categories and silos, but if it helps you organize and present your content better then it`s a safe way to design a Website. I`ve seen much worse ways to design them.
Kumar Sanjeev: It helps a lot. For building nice topical authority between inter-connected pages. Also Internal Link Juice has gets improved for important pages.
Razvan Alexa: they do, especially if you take the time to personalise the category pages with relevant text