Weak currencies have worked great for countries like China. It creates a lot of employment for export. It's a way to trick people into working at lower salaries I guess, a way to lower minimum wage yet give people the impression that it goes up (inflation has a similar effect).

More jobs has a strong political effect too; people will probably tolerate more austerity if it's not combined with unemployment.

The article explains pretty well in my opinion why that 3% is very capable of collapsing the rest of the EU. If Greece defaults, investors lose their money. That's not too bad (although there could be a domino effect with banks), however investors will also start worrying more about a default of Ireland and Portugal, followed by Spain and Italy. Such worry translates to higher interest rates leading to the horror scenario of Spain needing a bailout (which noone can afford to give them).

Anything that amounts to Greece not repaying their debts, will be considered a default by the market, likely leading to what I described in the above paragraph.