We had a pretty good discussion of the topic in this thread:
Text alignment in Sigil .
In brief summary, full justification was a source of pride in the typesetters guild. It took a lot of effort, and skill, to set the spacing on each line to fully fill the page. Any document that did not display this full justification was considered "substandard", "bad form", "second rate". That has been what people have grown up seeing when all the pBooks (books, textbooks, news articles, etc.) were printed that way.
The current computer age is used to seeing webpages, blogs, forums, chat, tweets, school reports/papers etc. all left aligned. Since "full justified" can be achieved through the click of a button - or at least a
form of full justification (they only put spaces between the words, not space the words themselves) - there really isn't an inherent pride in the work anymore. It is just what people (usually older ones like me) are
used to seeing.
Authors, and some eBook creators, in their attempt to reproduce a paper book on the screen will always justify their text.
I personally prefer left justification. I don't like the weirdly spaced words that can pop up in the flow of text with full justification. "At practical font sizes" full justification may not look too bad, but I would say almost all devices allow the reader to change the font....then your book MAY look bad. Left justification avoids that possibility.
Having said all that. Most devices/apps that I'm aware of let the reader choose the justification. There ARE some devices that do NOT override any publisher settings, so what you set COULD be what the reader is stuck with.
If you set full justification, or leave it unspecified (some devices default to full) it COULD end up making those erratic spacings appear. If you set left justification it COULD force readers to look at the "jaggies" on the right. Pick your poison...
Cheers!