Are you sure you're looking at the right site when you see, "Show falling snow on my blog until January 4th"? That's a default option available in WordPress.com sites.
The advantage of an XML sitemap is to help search engines find your pages better, as explained in the resources discussed in your first thread. It's an SEO feature. That could be said about any SEO feature and not just sitemaps.
What is the issue when people have to download an SEO plugin other than the dashboard gets "cluttered"? That's not a compelling point that you really need to convey in order to convince us there's a dilemma here.