here's an amalgamation of stuff I've read here, on blogs, and my own thoughts:
So the island itself is the bridge between life and death. So it is the physical manifestation of purgatory itself, in 'dark tower' of life and death in our world if you will.
If the island were to be destroyed, that bridge would collapse and souls would have no way of entering the afterlife. If Smokie was Smoke at the time the cork was pulled, he would have simply vanished. But because he was in Lockes body, MiB took the 'shell' of Locke and became a soulless, mortal shell.
So that's what would happen if MiB got off the island - not that he's [physical evil in itself, who would go around causing mass genocide (we never really got the impression this is what he wanted, and MiB as a human certainly didn't crave it) but it's that humanity loses it's connection to the afterlife, we all lose our souls and become mortal coils. We lose our humanity, and we become closer to what Mother was - unsympathetic, uncaring, and trapped. This goes along with all the super vague descriptions Jacob and Widmore give when everyone asks them what would happened if MiB escaped. It's not like anyone ever said he'd just go on a murderous rampage - things like 'everything you know will change, or this will 'end very badly,' or all you once knew is gone' - stuff like that
I like this train of thought because it makes the island not only destiny or important to these characters lives, it's also the literal lynchpin between the Sideways and Island 'timelines' we've seen Season 6. Also explains how a place like this can 'trap' souls like Michael.
