That did make dynamic creating and removing instances a bit tricky.
On the other side, AWS Lambda "cloud functions" are billed much more precisely.
So now this precision comes to AWS EC2 also.
New – Per-Second Billing for EC2 Instances and EBS Volumes | AWS Blog
"Effective October 2nd, usage of Linux instances that are launched in On-Demand, Reserved, and Spot form will be billed in one-second increments. Similarly, provisioned storage for EBS volumes will be billed in one-second increments.
Per-second billing also applies to Amazon EMR and AWS Batch
...Per-second billing is not currently applicable to instances running Microsoft Windows or Linux distributions that have a separate hourly charge."
(So for Windows VMs Azure still has an advantage)