Andy Mallon explains why granting ALTER operations to a user means that you’ve granted DROP permissions:
Dropping stuff is destructive. Anyone who’s ever dropped a plate or a car knows that. But dropping a table, procedure, or database makes it go away completely. The only way to un-drop something is to recover it from backup, or to re-deploy from source control (you do have all your code & schema in source control, right?). Getting back to the original question’s premise (it’s OK to change the object but not to completely make it disappear), I ask…does it even matter? What are you preventing? Is dropping an object worse than altering it?
Read on for Andy’s reasoning.