But to be fair if you enable both you're paying for 3 servers total instead of 2, you can't read from the HA standby. (I'd imagine there are reasons not to do that anyway, but you don't even have the option of making that compromise to save on the cost)
With RDS Aurora, any of your read replicas can be promoted to read/write master in the event of a failure of your primary master. This happens in 10-15 seconds, and is very fast.
So, you can get the benefit of up to 15 read replicas, and not have to pay for an extra standby server that is sitting idle.
Well, you can use the Read Replica only, and if you have an outage on the primary, promote the Read Replica to recover... (will take a few minutes though.)
