Well, it's mostly bad to not have a BOV on ANY turbo setup. That includes those with and without intercoolers. The Blow-Off Valve or BOV acts as a vital pressure release under boost + closed throttle situations. Now when you're boostin and you all the sudden take your foot of the throttle(shifting) you create a pressure "backflow". When you take you foot of the throttle, the thorttle plate closes and causes a pressure build up in the intake tract. Now with no where to go, the pressure rebounds towards the path of least resistance, IE the turbo. So when this pressure wave hits the turbo it causes the turbo to stop spooling, or even spin opposite it's proper direction........which is VERY bad seeing how the turbo is also connected to your exhaust tract, and would be basicaly trying to pump exhaust back into your engine in this senario........a BOV prevent's all this by opening when it sense's vaccume in the intake manifold and boost in the intake tract.

Cliff Notes: No BOV = BAD