In our organisation, we need to store fields that are calculated using the content of other fields. For instance, we need to create a date field which value is 2 days after the creation date.
Depending on the issue type and the project, the calculation rule is not the same: for bugs, it's "creation date + 2 days", but for enhancement it's "creation date + 5 days". It's something that should be automatic, because we cannot easily know what is the rule.
We need also to calculate integer fields or String fields based on values of other fields of the current issue.

In fact, the more flexible implementation of this should be to deliver the capability to define a Java class to define the default value of a field.
In this Java class, we should be able to use an API to access to the value of every other fields of the issue that is being submitted.
We should be able to associate a calculation class to a field at the field layout scheme level.