[jpatte]

The concept of commands is very useful but I don't think it belongs to the event sourcing pattern. Event sourcing is just a way to write and read data, it doesn't say anything about how you process inputs and how that processing can lead to a change of data. For that concern there exist other patterns like CQRS which introduces the concept of command. These patterns are complementary, and using both CQRS + ES is actually common practice.