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My rule of thumb is anything visible and on page should be optimised for the users experience.  If it's going to help them consolidate the knowledge that they've just got from your site then it is beneficial. It will not help you in climbing the SERPS, as link juice and other metrics does not pass backwards in a link.

You should try DYNO Mapper. It is a visual sitemap genreator that is good for mapping websites that are up to 25,000 pages. It is a paid tool, but their rates are pretty affordable. It generates sitemaps that include Google Analytics integration, content audit tools, and collaboration tools. 110649952

No offical testing here but, ANY thing can have behavioral SEO benifits that could lead to more organic conversions.  But do they have algorithmic value? Probally not, but I will conduct some studies into this in my free time. But I would love it if we can find some legitimate sources for this topic, digging initiated.

Go to the http://www.gumtree.com/ and create an ad for a house (or whatever), the in field help they give to get users to give indepth information is great without seeming forced. If you incentivise your community via "likes" or rankings, then they will want to provide this information i.e. make it in their interest to provide you with information