Meaning: I really don't need that
Rewrite 1: I need that like I need a flat tire
Rewrite 2: I need that like I need the dengue fever
Rewrite 3: That's about as much use as a flash flood in a Fizzies' factory
Rewrite 4: Sure, I need that -- right after I get a lobotomy
Comment: There are a lot of ways to say, "Believe me, I don't need that!" but the emphasis in this case is on sarcasm. So here are but a few ideas for recasting this classic cliché.

Notes:
Capitalization of dengue fever -- there is no consistency across several major newspapers, dictionaries, and medical sources. So I went with the simpler, as used in The New York Times.
What's a Fizzies? -- a candy that errupts into fizzes (thus the name) when dropped into water or other liquid. Imagine the mess with thousands of these during a flash flood while in production!