I'm always wary when removing objects that the server put there as well. The dt* objects appear to be used only for 'diagrams'. We are not using the diagraming feature in SQL Server, so we do not have a need for them. Yes, they get put back when you take the following action:
* Here are the actions in SQL Server Enterprise Manager which create the dt* objects:
* --> Expand Databases
* --> Expand a target database
* --> Click on Diagrams
* --> Right mouse and click 'New Diagram' (As soon as you do this, the table and
* procedures are added without prompting)
I have found no repercutions from removing these objects.
I think SQL Server is doing a dis-service by jamming the dt* objects into a user database without prompting the user they are about to create a pile of objects.