I have this question as well.
The current migration script feature is certainly useful, but sometimes we need to script updates/inserts/deletes that don't necessarily correspond to changes in the database structure.
An example, inspired by a recent case:
We needed to update a number of records in our Clients table and the easiest way was to write an sql script.
However, we didn't need to change the structure of the Clients table, we only needed to update some data. Thus, the migration script feature in SQL Source Control wouldn't let us add this script as a migration script.