This is mainly an opinion (and thus may get deleted).
- dark themes can be harsh on the eyes for some people if the text is very bright.
- dark themes can appear "gloomy" depending on the spectator
- energy saving, particularly on handheld devices
- less harsh on the eyes, particularly when viewed in dark environments
- stand out from the rest
For accessibility (i.e. visually impaired users) this shouldn't be an issue, so long as you ensure the contrast is high enough (this tool is useful for that http://jxnblk.com/colorable/demos/text/). One might even argue it is easier to see on dark themes. It could also be argued, that lots of visual work looks better when framed by darker shades, rather than white.
It is mainly a question of style. In recent years the "clean and crisp" look, found for example in Mac OS was considered the way to go. That being said, Mac just released a dark theme, so I'd say go with what you like.