Better late than never, I'll wade in with a defence of discussing multiverses since I've done a fair bit of that (this passed me by when you first posted it). Multiverses are a very relevant topic because it affects the kind of universe in which we would expect to find ourselves. In a recent topic I'd brought up the subject of whether we might expect physics to be simple or complicated, which is a very worthwhile issue, and the distribution of existing universes would have a bearing on that. Of course you cannot prove that there are many universes, but neither can you prove that there is only one. Which hypothesis is the more speculative? IMO it's the latter, and Occam's razor cuts the other way.
I've seen an awful lot of rubbish on the boards about multiple parallel universes (or "multiverses"), string theory, and other such drivel since I've been on here. I suppose some people are bound to buy that sort of bunk, just like there are bound to be people who believe in ghosts and fairies and any hope for the Steelers to win it all this year. But the mere fact that people go around holding such delusions isn't the disturbing part. What's really bothersome is that some people think this almost classifies as science. Absurdity! Multiverses are inherently unprovable. Could I go outside right now and run an experiment that could either definitively prove or disprove the existence of even a single parallel universe? Could you even theoretically come up with an experiment that could ever prove or disprove the existence of multiverses? No, I say!
So, since the multiverse "hypothesis" is an inherently unobservable and unprovable speculation that as far as I know or care to look up does not even need to be invoked to explain any natural phenomena, then off to Occam's guillotine with it! Henceforth, anyone who posts anything about these mythical multiverses must acknowledge the fact that there is no scientific value in what they're saying, and must use quotes around it to help emphasize the fact that it shouldn't be taken seriously, and must use air quotes whenever talking about it in real life, and must give equal time to every other postulate like the Flying Spaghetti Monster.
What precisely is the problem with an untestable hypothesis anyway? Your criticism seems to be based on the fact that untestable hypotheses are... untestable. But you have not ascertained that they are irrelevant. Take another untestable hypothesis, the existence of a god (for convenience I'll bundle the afterlife in with that). If there is such a thing, we would be wise to spend our lives securing an eternity in heaven, if not, we would be wise to live our lives to the full. Although untestable, it's hardly inconsequential. Incidentally, another reason why discussion of multiverses is relevant is in debunking the god hypothesis. One of the reasons for believing in a god is that our physics is fine tuned for life in a very improbable way. But it's only improbable if you make the assumption that this universe is all that there is; our version of reality has been selected to exist and all other possible worlds rejected. Why think that? Once again we have a choice between two untestable points of view which probabilistically have a great effect on unknown but important aspects of the world in which we live.
Untestable hypotheses may be very relevant in the choices we make. As Harry Callahan put it, "I know what you're thinking. 'Did he fire six shots or only five?' Well, to tell you the truth, in all this excitement I kind of lost track myself. But being as this is a .44 Magnum, the most powerful handgun in the world, and would blow your head clean off, you've got to ask yourself one question: Do I feel lucky? Well, do ya, punk?"