I don't think you understand how this works.
You are making a cash deposit into MtGox's bank account.
Last time I checked, most reputable banks will not let you "withdraw" money from somebody else's bank account. That's usually called a "bank robbery".
Maybe you know about some secret I don't?
If they were able to somehow restrict deposits to their account to only cash and not checks, then they're fine. But checks are, of course, cancelable.