Personally, I am a fan of having HTML Sitemaps - mostly for 404 like pages, so users who stumble on a page they are forbidden (sounds strict) to access, they have a way out but not out of your web site.
That being said, how necessary is it for Google and crawling? In todays day and age, Google does an excellent job finding content on web pages even if it is not linked to in a well-formed navigational structure. But HTML sitemaps can be handy in Google crawling your site naturally, not with an XML Sitemap.
The question came up in a Google+ hangout:
We know HTML sitemaps can be useful for users but is there any added SEO value in them if we already have XML sitemaps properly submitted to Search Console?
At the 5 minute mark John said:
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 links, that can really help us.
But in this case, it is more important you listen to how John answered it versus just the words he used:
Forum discussion at Google+.