Re: @Destroy All Monsters - "humans limited forever"
And furthermore, even if there is no FTL travel, that doesn't necessarily make it *impossible* for us to go to the stars, just difficult, and long. There are lots of possibilities for "hard" interstellar travel that don't require radical new physics: e.g. we could use genetic engineering to increase our life span, thereby making even a 50- or 100-year star trip feasible. We could do it via multi-generational "migration" trips. And in lieu of sending biological life, we could build A.I.s that could travel out there over the long centuries.
All the above are theoretically plausible, and given what we know, more plausible than FTL travel, even if we don't know how to do them yet. There's no _laws of physics_ that forbid them.
As for "spreading beyond the biosphere", no, it's not _required_, but I suspect that it'd be good for us if we didn't give up on our drive to _explore_ beyond it at least, even if it will take a long time to do so. And furthermore, we don't need even interstellar travel of any sort to "spread beyond the biosphere" -- setting up a base on the Moon would do. :)
Why go to space? Simple. We are opening up _new_ things, so that we can do _new_ stuff that has _never been done before_. It challenges us, and it is the logical continuation of our desire to know what's beyond the next hill and the next river. And we'll learn a lot doing it -- stuff that'll likely have many unseen benefits.
Though I'd think that for now, the issues on our planet are more pressing than going to the stars, but once we can overcome them, then, well, the stars seem like a logical next step.