Pretty sure Ashley himself says the square knot should never be used as a bend, and that doing so has probably killed more people than any other knot failure.
Square knots used as bends have a tendency to loosen over time when used with rope that is not exactly equal in characteristics, especially when they are not under tension (or the line alternates between slack and taut).
I believe the US Navy recommends 2 bowlines over a square knot, but the sheet bend is still the gold standard.