EC2 instances only have one network interface. The public IP address you have pointing to your instance is a DNAT done somewhere further up the chain.

If you get a large network load to your instance - say, a DDoS attack - you can find you no longer have enough network capacity to talk to your EBS disks.

This is what happened to Bitbucket in 2009: http://blog.bitbucket.org/2009/10/04/on-our-extended-downtim...