While I don't think our old mate Casper really understands what he is talking about, boost control through blow off valves is not quite as simple as how it was explained below.

It is true that you overwork your turbo to a degree through not allowing it to regulate it's speed by bypassing exhaust gas. However the turbo is regulated by the bov. This happens because the bov lets a proportion of the air that the turbo has just pumped, out to the athmosphere.This then means that not all the air the turbo as worked for will pass through the motor which equals less exhaust gas and hence less energy for the turbine(so it won't spin as fast).
So if you can get your head around that you'll see that the turbine speed is actually regulated. It's still not ideal but it's not as bad as it seems.