These Target-2 claims of the Bundesbank are actually claims against the ECB but if Greece exited the Eurozone, the ECB wouldn’t collect on their claims against the Greek banking sector and neither would the Bundesbank collect from the ECB.
De jure, the Bundesbank is only liable for about 27% of that amount; the other 73% would have to come from other Central Banks. That will be fun to watch when the Central Bank of, say, Finland (which may not have lent anything to the Greek banking sector) may have to come up with money because the Bundesbank has lent too much!
If Greece exited the Eurozone, the Bundesbank is likely to lose much of the 100 BN EUR. If the Eurozone breaks apart, it loses all of it (plus another 400 BN EUR of claims which it has to other PIIGS banking systems).
If that isn’t a good motivation to keep the Eurozone alive, then I don’t know what is…