I liked the original form of this idea better, when it was hypothesized that universes are packed into a stack-like structure of membranes, although the mirror hypothesis has the benefit of being theoretically accessible to experimentation.

There are a couple of neat effects that fall out of the (mem)brane hypothesis. For example, it offers a model for why gravity is seemingly so weak compared to the other forces: because a large part of it leaks out into neighboring universes. In that case, we'd have a handy explanation for dark matter as well: it's gravity that leaks in from neighboring universes. This would mean that stuff clumps together across universes. It could also be an auxiliary model for black hole formation in certain cases where we have yet to find out how some black holes could grow very quickly.

If we ever become a K3 civilization we could attempt to send a message across universes with gravity, or possibly even paint a symbol into the sky using large masses. Not we know how likely the other universes are to support life (probably not very).