Of course the validator doesn't care because it doesn't make the distinction between HTML and XHTML. Using the / implies that you are writing XHTML 5 rather than HTML 5 is so far as the (X)HTML 5 validator is concerned.
When served as HTML the / is invalid but the error correction that browsers apply to HTML means that the error will be ignored (unless the browser is Netscape 4 in which case the / will invalidate the preceding attribute unless there is a space in between).
When served as XHTML the / must be there or alternatively a closing tag included in its place eg. <br></br> is equivalent to <br/> in XHTML (including XHTML 5). If the / isn't there and there isn't a corresponding closing tag then the page will not display proeperly.
(X)HTML 5 takes advantage of the fact that browsers will ignore the error when the / is included in HTML to avoid needing to distinguish between the two. The same standard can then be applied to both HTML and XHTML making it easier to maintain the standards for both languages.