Extensions (like .html) should be considered only from Web Servers point of view & not for URL scheme perspective. And if Web server is capable enough of recognizing URL & resolving it to the appropriate resource (without file extensions), they should be avoided. To assist the URL resolution on Web Server end, MIME type should be properly configured.

On CMS side, SEO should be the center point while deciding URL scheme & file extension doesn’t bear any impact on SEO friendly URLs. Next, from technical point of view, tomorrow if there are changes in technology and “html” extension are replaced by some “xyz” extensions, you need to build up the complete hierarchy and fix SEO issues. Persistence is the key and URLs should not change based on file extensions.

Another major advantage of having without extension in URL is that they are easier to read/remember for the visitors and look friendlier in the search results.

You can also implement extension less urls through redirection rules @ .htaccess level (on web server).

Also refer W3C paper http://www.w3.org/Provider/Style/URI to support this.