Is there any reason to avoid using text-align: justify;?
Does it reduce readability or cause problems?
Like any web design question... it depends. As has been suggested, justified text does not usually end up looking good in narrow columns. I'd recommend against it as a general rule in sidebars as sidebars are usually narrow.
With body copy at the often recommended 10 words per line (or with any amount above that) you will probably be able to get away with justified text and have it look reasonably good most of the time unless you're using a lot of really long words and/or strings.
I think I've pimped this site on StackOverflow before, but Jon Tan uses justified text in his body copy (within articles) and it looks great 99.99% of the time.