Even if untestable, the Multiverse Hypothesis is immensely valuable in that it serves as a rational replacement for religion. It gives people who have a certain mindset something to believe in. And not just any something, but something exciting that inspires the imagination.
Testable or not, here is the thing. The hypothesis being not testable means that it cannot be shown true, and that it cannot be shown false, either! Moreover, hypothesis is quite persuasive: more so than its negation, to many people.
All arguments against the hypothesis based on testability ("you don't have proof") are also weapons against the negation of the hypothesis. They do not attack the source of its plausibility; they only attack something which it has in common with it negation.
Both the hypothesis and its negation postulate the existence of something: one postulates the existence of an infinite plurality of universes, whereas the other postulates the existence of hidden variables and inexplicable coincidences.
So, even though belief in MH seems to have some of the trappings of faith, it is at the same time rationally convincing to those who find hidden variables and coincidences less plausible than a plurality of universes.