I sometimes find myself needing to connect to any instance in a particular ASG, and I don't really care which one. This tool is built to solve that particular problem. If I want to access a specific instance I will address it by its instance ID (ssh2ec2 --instance-id i-123456).
What would the index refer to? The list of instances in an ASG would need to be sorted (perhaps by instance ID) in order for an index to be useful. The behaviour would still be undefined to a certain extent, as the logic would be "return the instance at position n in the list", when the number of instances in the ASG might have changed due to a scaling event.
Edit: Thanks for the pull requests - we can discuss further there.