Jake Archibald has compiled some very good points against Data URIs at Google+.

With base64 the image data is in the css file, meaning it gets downloaded even if it isn’t needed. Eg, if you’re using media queries to optimise image usage for mobile, base64 doesn’t help.

The browser has to download the full CSS file before it can render anything, having imagery in a separate request gets you progressive rendering, whereas with base64 the CSS and image data has to download before anything is displayed to the user.

— Jake Archibald - Google+ - I was asked about base64 vs a sprite image in CSS