HTML5 is a huge improvement over the HTML4.01/XHTML madness that was going on back in the day. And it's fine with me to allow non-closed singleton tags.
There's perhaps no strong logical argument either way, but from a style perspective, I prefer to use closing slashes to make it absolutely clear what's going on.
Allowing the void tags to be unclosed is the lesser evil of the two, I can even accept the argument behind it (they can't have content) even though it complicates the syntax.
The really evil one is to not make <div /> be exactly equivalent to <div></div> which is just batshit crazy. When I want a placeholder tag (to be populated later) I have to write <div></div> which feels completely unnatural,
There's perhaps no strong logical argument either way, but from a style perspective, I prefer to use closing slashes to make it absolutely clear what's going on.