I'm not sure what one currency or another would have to do with it- if they're one 'diplomatically', one diplomat represents the interests of the one EU.
I imagine a unified Euro block with more than one UN seat would leave open the possibility for those seats to express different opinions and thus become a source of divisiveness within the EU. But I don't know, maybe the currency thing is really that important to them.
diplomacy would require the unity of one ecconomic power. That is if they were to have one, legally binding for all 25, voice. This would allow ecconomic sanctions/rewards to be more efficiently used. It would also make it far more possible to make the euro the world's reserve currency and the eu's bully stick.