MIB, as I understood it, was represented as the wine in the bottle, and the Island was the cork. Jacob is there to make sure the cork stays in place because if evil escapes then all hell will break loose, like Hurley said.
Basically, after being trapped in his cell for so long, MIB doesn't care, but the effect of his leaving the Island to the "real" world would mean his evil would spread, much as it is doing on the Island.
As messed up as this world already is, imagine how much worse it would be with a bunch of Roberts, Claires, and Sayids running around, all changed by the "infection" of evil.