When adding data conversion it would be great to have other options than only adding a constant default value.
For the develop environment this may be sufficient, but when setting it through to other environments you lose the option to look into the database and see which constant value is appropriate.
Being able to have expressions you can at least determine an appropriate value. Being able to add a script, you even have the option to add a dummy record and reference that one. This particularly when a foreign key field changes to mandatory.
