The idea of parallel universes is not Hawking's. He may talk about the idea in his popular writings, but in scientific circles the concept is not associated with his name.
The idea is not a theory. It is a hypothesis. As far as I know, it does not synthesize hitherto disparate facts or predict testable consequences.
The idea is not ridiculous. In fact, depending how you define the term "universe", the existence of parallel universes may be strongly demanded in light of the WMAP data that was released in February. These data prove that the universe is flat out to the edge of our Hubble volume. The simplest interpretation is that there exists galaxies, stars and planets beyond the edge of our Hubble volume, and that in fact this extends infinitely in all directions. If you define "universe" to mean "everything we can geometrically travel to or see" (as most physicists do), then there exist a gigantically large number of Hubble volumes--universes--similar to ours, lying beyond the edge of our own.
The alternative--that some unknown physics radically alters the geometry of spacetime out beyond where we can see, and cuts it off somehow--is a radical idea without a shred of evidence, and which is based upon nothing but a desire for physical reality to be other than what it clearly appears to be.
I'm sure it was unintentional, but layman reading the above are going to draw the wrong conclusion from your remark.
There is, of course, no physical "edge" to our Hubble volume, and the Universe (in the sense of the union of all possible Hubble volumes) is unbounded and infinite, as per the WMAP data to which you alluded.