They are actually not trying to defend the euro currency directly, how would you defend a currency by printing more of it? IF they wanted to support the exchange rate of the euro they would have to sell dollars from their reserves and buy euros on the open market.
No, what they are trying to do in this case is guarantee the long-term solvency of the member states, which they believe will translate into a more stable currency.
But of course, this plan will increase the fiscal burden of everybody in the eurozone, so I think the euro will continue to stay under pressure even if the immediate pressure is taken off the PIGS.