[cmaury]

Justified paragraphs are mostly useful for text laid out in columns (i.e. Newspapers and magazines). This way when you are reading the second and subsequent columns, the eye can easily find the start of the line/column.

Most blogs are single column. Justifying the text will create variations in the space between words which makes it harder to read with little benefit other than an overall aesthetic that may or may not be more pleasing to look at as a whole.

[whoopdedo]

But many blogs will place advertisements or other elements to the right of the main content. Would an aligned border improve the readability of the ads leading to higher conversion when text is justified?

Alternately, you may get lower conversion as the aligned text creates a "stop here" signal that makes the reader less likely to wander into the ad space. Should be worth A-B testing.

[jacobolus]

Justified text really only looks good if either (a) you have correct hyphenation and a high-quality paragraph composition algorithm, such as the one used by TeX or Adobe InDesign, or (b) you lay the text out by hand very carefully for a fixed-width layout. To make justified text look really good, the start and end positions of each line need to be optically adjusted (in other words, vary a little bit from line to line to account for varying letter shapes) and usually punctuation stuck slightly into the margin.

In a web browser, text composition is done line by line (because that was more practical for the compute resources available in 1995, and no one has bothered to implement the better alternative, even though every device used today has more than enough CPU to handle it), there is no hyphenation, and you can forget about optical adjustment. As a result, justified text invariably looks like complete garbage, with dramatically varying amounts of space from line to line. These turn into distracting “rivers” of whitespace running down the page.