Should I avoid using "text-align: justify;"?
Is there any reason to avoid using text-align: justify;?
Does it reduce readability or cause problems?
Answer
Firstly, this is purely a design-related problem and solution. The design of your grid specifies if justifying text is needed. I think justify align alone has no major effect on usability. Bad typography that makes text illegible is what decreases usability. That said, make sure you have solid contrasts and a good balance of spacing in your type. Font-size matters too.
This site is a successful attempt at justifying text online. Since you can't control the spaces between letters and words nearly as much with CSS as you can with InDesign, it's a lot harder to justify text and not have 'rivers' of whitespace run down your rows, which happens when the spaces between words exceeds the spaces between lines. Things that make justifying text difficult: long words, row widths that don't hold enough words, and too much contrast between the text and the background colors; they make the rivers appear wider and more apparent.
The typographic ideal is to have an even text-block, where if you squint the text block becomes a uniform solid shade. So if you keep text at about 10px and 60% gray on white background, and have rows that fit about 20 words, justify align shouldn't look ugly. But keep in mind this does nothing to improve usability. Small, relatively light text annoys a lot of people, especially the older readers.
I should note that with ­ (soft hyphenation) correctly separating long troublesome words like superawesome you can mimic proper typesetting (e.g. InDesign). There's a lot of stuff out there to help you do this. I would recommend the logic for hiding, manipulating, and displaying the text to be done on the client side, perhaps with this.
Edit: As others mention below, css hyphens are possible now, except not feasible (not in Chrome). Plus, css4 offers even more control.