I tell students that there are only two valid reasons for centring like that: fancy wedding invitations, and fancy restaurant menus. If you look at either, and squint, it's like a Rorschach Ink Blot Test.
What's more, it's difficult to read. When your eye gets to the end of a line it has to do a little "carriage return/line feed" operation where it moves down to the next line and back to its beginning. With a strong left margin, your eye knows exactly where to go to. With ragged left, it has to figure it out for each line.
Sadly, to the uninformed, such abominations as script fonts, full centring, and "misused" "quotation" "marks" mean important, fancy and elegant.
That said, I LIKE when blogs or websites centre a table in the middle to restrict line length. Maybe that's my 20" high-res monitor speaking, but I don't like reading as though I'm watching a tennis match. Hard on the neck. :)
ed'd to add: I'm looking at Judy's blog, and it's left justified. (??)