My understanding is that a number of nations sit somewhere on a spectrum between "solvency" problem and "liquidity" problem. Difference being, you can grow out of a liquidity problem as long as you are given time.
Greece is on the extreme, and looks like a solvency problem. They arent large, so a default makes sense for all involved. But if the European community allows a default to occur (and just capitalize banks as needed)... They now reset all of our expectations about other peripheral nations (Italy and Spain for starters), which arguably are only facing liquidity issues.
Arguably these other nations COULD grow out of their current deficit given time... but if investors flee their bonds while speculators pile on, then their borrowing costs will skyrocket. In other words, by not defending Greece, these other nations would also become insolvent. And at that point, their combined size is likely too large for the remaining richer European nations to save.