When you loose condenser vacuum, you certianly loose turbine power. This means that you are no longer "sucking" the steam through the turbine as much. The steam, therefore, is hotter as it comes from the steam turbine. It is no longer saturated steam, but it is superheated steam, and carries with it hotter conditions. This means that there could be destructive problems with turbine blading ... materials issues. blading is already designed close to failure, so this higher temperature could cause blade seperation and possible catistrophic failure of the turbine and condenser. And when you loose one piece, you will usually loose more than one. Remember, this blade is rotating at high speed, and at a large enough radius from the center of rotation that it will contain a lot of destructive energy.

So, to make a long story shorter, This needs to be avoided for equipment long life, and the avoidance of catestrophic failure ... besides the wasting of energy, loss of efficiency, and loss of power.