In CRM 2015, when you create a new field, you can choose the type of field as calculated or simple. This feature is available everytime when you create a new field, with the exception of primary field. Technically, a primary field is created when you create the entity, hence the user should get the same capability that is available to other new fields.
Lot of times, I am write script to concatenate couple of fields and set the primary field as I make the primary field optional. Adding this capability would reduce time spent on writing such boiler plate code.