"When I introduced tangible and shadow photons I apparently distinguished them by saying that we can see the former, but not the latter. But who are ‘we’? While I was writing that, hosts of shadow Davids were writing it too. They too drew a distinction between tangible and shadow photons; but the photons they called ‘shadow’ include the ones I called ‘tangible’, and the photons they called ‘tangible’ are among those I called ‘shadow’.
Not only do none of the copies of an object have any privileged position in the explanation of shadows that I have just outlined, neither do they have a privileged position in the full mathematical explanation provided by quantum theory. I may feel subjectively that I am distinguished among the copies as the ‘tangible’ one, because I can directly perceive myself and not the others, but I must come to terms with the fact that all the others feel the same about themselves.
Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea."
And his summary:
"the whole of physical reality, the multiverse, contains vast numbers of parallel universes"
So no, Deutsch, who is arguably the leading advocate of MWI, does not consider parallel universes to be a "metaphor/model/aid to understanding". He insists that they are real. He even defends this position at extraordinary length in chapter 10 of TFOR.