If scientists and science enthusiasts took the content of their beliefs seriously things would be very different. In the Age of Laplace they would have realized that, all things being determined, it wouldn't matter whether people believed in scientific progress at all: beliefs only have causes, not effects, and so the great efforts to rid the world of superstition and make it comfortably secular were pointless, if inevitable. Why argue that we ought to adopt science as our guiding light if there is no such thing as guidance at all, if instead all we do or do not believe--and argue--was pre-established in some dark and forgotten aeon? In the Age of Carnap, where the reduction of all thought to observational atoms ruled out the normative charge of any thought, why work towards a socialist future as so many of the Logical Positivists did? There could be no 'good' socialist future over and against a 'bad' capitalist past, because there is no 'good' and 'bad' in the first place.
But, just for the sake of maximal generosity, let's grant that there is a multiverse. What are the practical upshots of this? How will this inform our everyday? I submit things don't fare much better than with Laplace: why do anything, if the very fact that we could means that in some other universe someone will? In fact, if we do x, then those in that universe will then necessarily not do x, and we will thereby deprive them of what we have--what foul usurpation! Or, not wishing to neglect the poor and downtrodden of other worlds, we could forego doing x in an act of charity to our multiversal neighbor . . . but then that would preclude his option to be charitable to us, right? Or am I committing an error in assuming that choice here precedes and precludes choice there? Or more likely, do I falsely assume that there's anything like choice at all either here or there in the first place? I mean, if every path is taken, then every path is taken period. My own desires, or the illusion that I am taking action, has nothing to do with anything. Which means, as with Laplace, that there is no reason at all to promote Multiverse Theory or pat ourselves on the back that we were clever enough to come up with it in the first place. Which means that Tegmark & Co. are caught up in performative contradictions. Unless, of course, you grant that they can't help it, as it be the dispensation of some dark aeon.
But let's take one more stab at outdoing ourselves with charitable interpretation. Let's grant that there is a multiverse and that Tegemark has done us a favor in bringing it to light. Does this mean that science has conquered all, and forced the last vestiges of superstion from our midst once and for all? Well, if the multiverse is, then the multiverse is, which means that each universe in the multiverse is, which means that the Problem of the One and the Many is still very much alive. So much is metaphysically indefeasible. Whatever the advantages or distractions of empirical progress, we still have to know how being and plurality can both be true, when prima facie the only thing to cause plurality of being would be an addition to being from outside of being . . . and outside of being is nothing. Which puts us with Parmenides at the headwaters of a very different tradition . . .