IMHO, migrated questions kept on the original site has its benefits. One of which is making it searchable on the original site.  Completely deleting the question on the original site allows the same question to be asked again on that site.  If the original question is kept and is pointing where it is migrated, at least it can be flagged as a duplicate of a question and then point it to the site where it is migrated.

**Edit** to answer Kyle: The examples are good to show that some of the question ought to be deleted.  But I think not all users should be allowed to delete their own questions because it's not only them who may benefit from the question, but the community as well. I think it's just right that those who have more power/rep be allowed to delete their own questions.  

Although it would be nice to allow the flagging of the migrated question to be deleted. I have that similar case where my question is an exact duplicate but accumulated some answers so I cannot delete it.