Although I most certainly don't support this pointless waste of money bailing out bankrupt nations truly is, there are substantial dangers in not only allowing these to default, but to have them leave the Euro as well.
First off are the well known direct implications with regards to the owners of Greek government bonds. Whoever hold these - banks, retirement funds, hedge funds - will eventually need to accept a significant haircut (writedown on the value of the bonds). Already, yields are reaching astronomical figures, a direct result of no-one wishing to buy Greek government debt, thereby effectively reflecting an expectation of an upcoming default.
But a far greater problem it could be to allow the Greeks to leave the Euro. In the event this were to happen, the Greeks wouldn't actually need to formally declare bankruptcy - they could instead forcibly convert their bonds to Drachma denominated, only then to let the currency crash. It should be rather obvious this really is no different to actually declaring bankruptcy, as for the bondholders the net result is the same - a significant haircut.
But the implications of letting the Greeks perform this covert bankruptcy stretches beyond the Aegean Sea. It would in an instant leave the bond market for other borderline bankrupt nations completely frozen - if the Greeks can leave the Euro, stuffing the bondholders in the process, why wouldn't Portugal and Ireland follow in their tracks? This would case yields on not only currently troubled nations such as Ireland and Portugal to go parabolic, but also - from a point of view of scale of the respective bond markets - far larger nations such as Spain and Italy.
And if yields for Spain and Italy were to skyrocket, it could become an almost overnight self-fulfilling prophecy - these nations could be forced into bankruptcy, as the bondmarket no longer would have any faith in these nations staying in the Euro, the very thing the European Central Bank and the IMF have been attempting to avoid.