In addition, left alignment of text allows for easier scannability. Similar to how there's fast recognition of words based on word shape, I believe the same holds true for paragraphs. It's easier on the eyes to peruse down with a ragged right edge.
Justified text is good from a print perspective because it regulates the shape of the copy texture for the designer and is easier to implement due to the meticulousness that's needed to set type.
On the web, there are too many variables when it comes to digital "Substrate" such as screen size, accessibility, browser type and zoom level that can have a real impact on the exactness of doing justified text properly.