If they devote less of their economy to debt service (my understanding is that this is why they would leave the Euro), then they can have higher consumption at a given level of economic activity.
(I don't really understand the situation well enough to say whether or not they could possibly maintain the same level of activity outside of the Euro, but I think this is the logic behind that argument)
Poking around, it looks like Greek debt service as a percentage of GDP is not particularly high (it's closer to France's than to Spain's). What does Ireland's interest/GDP look like? Iceland? Those are two former basket-case economies that seem to be recovering without total bail-outs.
(I'm asking, not telling; I don't know what the answer is.)
(I don't really understand the situation well enough to say whether or not they could possibly maintain the same level of activity outside of the Euro, but I think this is the logic behind that argument)