The monetary impact of a default in Greece should be very small in the Euro given that the percentage of GPB of Greece is very small compared to the rest of the Eurozone.
However such default would mean that a large institution as the European Central Bank is unable to deal with a small problem like is a hypothetical default of the Greece government. This would lead to a downgrade of the qualification of all fund denominated in Euros such are the sovereign debt of the countries in the Eurozone.
This downgrade would mean less confidence in the ECB what is one scenario that would create a negative impact in the Euro.
What could be the consequences of such impact? Probably the Euro would lose some of it's value what is bad for the imports in the Eurozone. You have to pay more Euros for the same value of good or services purchased out of the Eurozone.
The impact of the exports would be positive as every good or service sold by countries in the Eurozone would turn out to be cheaper without any substantial change in the productive model in the UE.
In my opinion, keeping an Euro at a value in the range of 1.1$ 1.2$ is one of the current objectives of the ECB. Political leaders tend to emphasise whatever they can do to benefit the directives given by the ECB.