Quoting Felici, The complete manual of typography, p. 128:
[T]he gutters between columns have to be in tune with both the texture
of the type---particularly the leading---and the widths of the margins
[...]. The page should appear as a harmonious whole, and the columns
should clearly relate to each other to create a sense of their being a
unified entity, albeit in several pieces. If the gutters are too wide,
the columns look like a series of unrelated text blocks. If the
gutters are too narrow, they begin to affect readability, as the eye
takes in part of the type from the next column every time it ranges
toward the end of the line.
On p. 129, Felici shows an example of justified text columns with too wide gutters and notes "If the text were set ragged right, the effect would be even worse, as ragged margins make gutters seem wider."
In my opinion, the "optimum" column width is about 1.5em to 2em, and the value of
\columnsep in the standard classes (10pt) is so small as to affect readability.
Note: The following examples use the default font size of
10pt. As the standard classes don't enlarge the value of
\columnsep for the
12pt sizes, their readability is affected even worse.