NBH, for one- there' s no clink when you connect your release.
For two- there' s no damn little screws to strip out and loose if they ever hit the floor. (try buying some around town)
For three- the string loop is more forgiving if you should torque your release when shooting.

The reason I know all these things is because my bow tackle box has 3 metal " d" loops bouncing around in the bottom of it. Once I figured out how amazingly simple the string loop was I switched all three of the bows I had rigged up at the time.