Good points, Barty.
I personally prefer ragged-right (text-align: left; ) on the Kindle. Full justification looks good, most of the time, but when there are short lines it's trying to stretch out, it can be hard to read. Therefore our "house style" had been to enforce left-justification.
That said, the world seems to be divided about equally between those who prefer full and those who prefer left. I see no harm in allowing a choice, and I will do so in the future where possible. As you say, one can already vary the font size, and beyond that an e-book, being flowable, ought never attempt to precisely mimic a p-book. If you want that, use PDF format (ugh!)
Of course, things like "text-align: center;" have to remain.
And in a weak attempt to move back on topic, I will confirm peartree's observation that the "text-align: " property, where present, overrides the Kindle's justification setting.