It's not really necessary because the migration methods run only once. You might want to approach the problem from a workflow perspective rather than adding a DropTable method. Here's who I do it:
Create a rehydrate.bat that:
- Drops and recreates my db
- Runs Orchard setup via command line (this creates the orchard db)
- Runs my custom recipes to enable specific modules, import static/initial data
When I make major changes I run rehydrate.bat and I get a fresh site that is built from scratch. I have never needed a DropTable (if exists) method.