Miami should never have signed Alston to play ahead of Mario Chalmers. Chalmers has always been the best starting point guard + best complement to Dwyane Wade for the Miami Heat.
The question is who is a better backup PG -- Alston or Arroyo? -- and I think Arroyo is the better choice because he's more of a ball dominant scorer/playmaker and that's exactly what Miami needs off their bench. They're light on firepower and can benefit from having a playmaking perimeter player on the floor when Wade is not.
Instead, though, they start Alston or Arroyo and use Chalmers as a backup ... which is a bad use of players. It doesn't maximize their talent levels.