According to [AWS](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-groups.html) placement group is required to be able to utilize 10gbps network performance. Given that instances are required to be placed in same availability zone for placement group, this sounds risky. For ex, unavailability of one availability zone would bring down the infrastructure.
Considering the risk, why would some use placement group?