> Are you going to choose to crash into something because it's wasting fuel to vent part of the steam?

Under emergency you will vent, but under normal operations where you just want to slow down you care very much about efficiency.

Not just for monetary reasons either (though that's a factor) but because the lumps of coal you're not burning now could save your ass in bad weather and make you reach safety instead of being stranded in the middle of the ocean.