I don't see how that is unreasonable; Hawking's idea is a hypothesis, not a theory, as it has yet to be validated by evidence. However, it is potentially compatible with our current level of understanding of quantum physics, where matter can indeed come from nothing, albeit with certain restrictions.
Be careful; he's not implied that this happened at one time, as time itself is just a property of the universe. More that existence itself is a state of nature, and fluctuates between 0 and 1 at the quantum level. It's the sum of properties that is zero, but an 'unbalancing' of zero can lead to equal positive and negative properties, e.g. matter and antimatter. Energy itself is likely to be a property that sums to zero. The universe has indications of being much more multi-dimensional than our 4-d space-time, and indeed other time-like dimensions may exist.
As we know that virtual particles can come into existence from nothing, it's not inconceivable that our physical universe is just a larger scale version of the same phenomenon, and that al the properties within it (or possible corresponding multiverses) still sum to zero. In a way, matter is just an unbalancing of nothingness.
So while we cannot say this is the case, it's certainly conceivable, and is consistent with what limited evidence we have so far. This contrasts with the idea of a god, which is supported by absolutely no evidence at all. There's not even any formal hypothesis of a god, as nobody is coming up with any precise, formal definition of what a god actually is.
Remember; the Universe is the 'everything' - it *includes* any gods that exist. While some will argue that a god can create a physical universe, that's only a subset of the total; it avoids asking the bigger question: why does this totality (including any gods) exist?
Hence any god idea doesn't actually even *try* answer the question of why the 'everything' exists anyway, while quantum physics does. Trying to answer the question completely, using evidence, is more reasonable than deliberately avoiding part of it and using a zero-evidence presumption.
Edit:
@The_doc_man:
You're quite right about those hypotheses (M-theory, colliding branes) in terms of explaining our physical universe, with possible precursors or non-temporal states of phase.
However, it's still a valid question to ask: why isn't there nothing? What is the nature of existence? I feel that quantum physics is still the closest to getting to the nub of that one, and the virtual particles ideas is a big clue. It seems that Hawking is more trying to address that than a more localised explanation of our physical 'big banged' universe which may indeed be a subset of a greater maximal Universe.
In any case, the approach to understanding this must be based on evidence, otherwise we cannot tell the diference between any idea being valid and invalid. Hence we don't jump to conclusions, e.g. gods.