Certainly, however we seem to disagree on the fact that a full collapse of bonds of all PIIGS countries is likely, which it most certainly is not simply because Spain and Italy are at this point not insolvent and in the spirit of "if shit hits the fan we will just change the rules (read: print money)" there is just no way nobody will stick his figurative finger in the dyke before anybody drowns.
A haircut of PIG (all together in the worst case) is not enough to make a significant impact on any global market except for a very temporary risk aversion shock possibly. Although even that I just don't find likely. The past weeks have seen so many press releases and rumours about Greece upon a lot of which we saw significant dumping of risk-related assets (in the midst of a correcting/trendless market) so I doubt there is anyone left to hit the sell button once Greece finally does default/is forced to restructure. In fact I think a lot of people are going to hit the buy button at that point just because it means there won't be anymore pressure on the market from Greece going forward. I'll agree that a haircut of Spanish or Italian bonds would be an actual problem, but a default of PIG is not enough to push neither Italy nor Spain into insolvency so this situation would require a further worsening of the underlying economic fundamentals, which would require something unforeseen considering those are still pretty good.
Of course I am not privvy to what derivatives banks have on their books and if it turns out that some systemic institutions have been fucking up (again) by being heavily short Greek (and other peripheral) CDS and becomes insolvent, then all bets are off and that would be worse than Lehman possibly. This is a tail risk however (albeit a fat tail) and it's more likely they will not have systemically dangerous excess exposure further than their bond holdings.