The only advantage would be in terms pricing, only if you have the servers that are sending email in the same EC2 region as the SES region. In other words if both of them are in the same region, you wouldn't get charged any "OUT" data.
Otherwise you would get charged $0.02 /GB . If you look at the AWS EC2 pricing here: http://aws.amazon.com/ec2/pricing/ , it would be here:
Of course, if your clients are outside Amazon you would get charged "IN" and "OUT" data.