To be honest, I don’t remember the last time I opened an HTML sitemap but the truth is that HTML sitemaps related to your website should be considered in terms of web crawling and Google search appearance.
The first plus that always comes to your mind when talking about the necessity of an HTML Sitemap is the useful purpose that it will bring to some of your websites’ visitors. In my opinion it is mandatory to have a sitemap of your website if you have a more complicated type of structure.
It will bring value when, for example you have web pages that are connected only via search forms and there is no visible logical structure. With sitemaps you can also crawl through the website essentially, without needing the use of XML.
We all know that at this point in time Google does a very good job at finding content on websites and this is even if they don’t have a decent navigational structure.
With the usage of HTML Sitemaps you naturally map your categories and your detail pages. So in this line of thought it is a good idea to have a sitemap and some people will truly appreciate it.
This topic came up in a Google+ Hangout and John responded with:
Any SEO value in HTML sitemaps? Sometimes. It can definitely make sense to have these kind of HTML sitemaps, which are essentially a mapping of your category and your detail pages. Especially if we can’t crawl a website normally otherwise.
So if you have a really complicated navigation structure, maybe if you have pages that are almost connected just through search forms rather than a logical structure, then at least having one place where we understand the structure of the site, based on the link, that can really help us.
So you see, HTML sitemaps might give you a little bit of extra edge.