Yes, these things are probably liquid and hedged to mitigate risk. However, during market dislocations, that liquidity can evaporate quickly and a hedge is only as solid as the party on the other end of that trade.
During normal market conditions, there is generally not a problem. It is when one of those "unpredictable events" comes along that markets can lock up. If the past few months are any indication, it is possible we may see such an event in the not too distant future.
After the LTCM debacle, the NY Fed no doubt keeps tabs on both gross and net exposures by large banks.
But the risks going forward - way overbought/overvalued equities, a derivatives market whose complexity and scale is literally unprecedented, a global recession, currency union in Europe, and the inevitable pending retirement of Alan Greenspan - will stress the system in unforeseen ways.
*I* am mostly in cash and AAA munis....