In the Ext.data.Writer there is a writeAllFields config option. If it's set to false, only the updated fields of the models got propagated to the server side. This is very useful, cutting on the amount of data transferred, and making server side updates more easy and efficient.
The problem is, that in some cases even though writeAllFields is set to false, some fields still might be needed to get propagated to the server side.
Consider an optimistic locking solution, where the last version id is propagated to the client from the server side. As the data is saved back to the server, the server side must know the version id of the record to be updated to be able to identify dirty updates. If this data does not arrive, this task is not possible.
I'd suggest having a config, that makes the field always persist, independent of the writeAll config setting of the underlaying writer class.
But it would be better to have it in the framework, so everybody could use it the same way rather than having it implemented by everybody needing this feature... Guess that's one of the goals of feature requests.