1) some other countries got into trouble when they started bailing out their banks. Are you saying they should have just let the banks gone bust? Or that they should have bailed them out and then defaulted on those debts? Because the latter would have ruined not just the governments credibility but also that of its respective financial sector
2) if a country defaults, it screws over a string of banks, pension funds and governments in other EU countries. I can see how this would be beneficial to the defaulting country in the short run, but why should the rest stand for this behaviour?
3) just speculation: let's say a country has about 100% GDP sovereign debt, and it defaults on half of it. If that causes the interest rates at which they can refinance their remaining debt to double or tripple, how is that country any better off? If a country defaults on all of its debt, can it manage to run a positive budget for the next couple of decades? Because a lot of people will be pissed, and simply not lend at all.