DEAR CRABBIT: Can I pitch an unfinished book to an agent?

Thanks for your questions for Dear Crabbit so far. Keep them coming! For guidelines, see the Dear Crabbit page above.
You asked:
"I have read your book (AKA my bible) Write to be Published and I have one small query: I understand why it is wrong to send a publisher an unfinished book (i.e. you are offering them a product, which may never get off the drawing board and therefore potentially