[Ginola]

To the question why is it bad? The basic answer is that it causes over fueling issues, the MAF sensor reads how much volume of air passes into the intake system and fuels accordingly. A recirc valve, when the throttle is lifted, opens allowing excess pressure to pass back into the inlet in front of the turbo so the air can be re-used again and the turbo draws it back in as you depress the throttle again, the measured quantity of air remaining the same as the MAF has measured. With a dump valve you vent the excess to atmosphere but the ECU does not know this nor does the MAF, fuel is fed in as if that volume of air is still in the system. This can of course be mapped for! But it's not set up for it out of the box.
As for blanking it off for a nice noise, you can do this but you will shorten the life of your turbo by making excess pressure stall the turbo when you lift off the throttle. (Not good).. But it sounds nice!

So basically leave it alone unless you need one. Standard cars that are not re-mapped will drive better with the standard valve with better throttle response and smother gear changes etc etc.