The Sheet Bend would replace the Square (Reef) knot except that it is not a binding knot – both ends must be loose in your hands with no load on the ropes. (The Square Knot - with all its faults - can be tied tight against a sail, or parcel, and usually stays tight while the second Half Hitch is tied).
And from http://www.animatedknots.com/reef/
Caution: This picture demonstrates how even a "Stack" of Square Knots can capsize and pull undone. These photographs were created by pulling on the ends of the red rope. "There have probably been more lives lost as a result of using a Square Knot as a bend (to tie two ropes together) than from the failure of any other half dozen knots combined." (ABOK page 258). Never use it for critical loads.
So which knots are alternatives to the reef knot when there is load on the ropes that solves this insecurity?