For a smaller charger that supply is fine. But it simply won't power a larger charger like the Duo to full power. If you don't need the full power then again it's fine. I actually use a slightly smaller supply than that on a Duo right now, but I will need to upgrade if I jump to larger batteries or higher charge rates.

Not to mention many of us power more than one charger from a supply, and other things besides chargers.