The purpose of a DV OR BOV is to save wear on the turbo.
When you close the throttle, all that boost pressure has to GO somewhere. If it doesn't it slams back into the turbo impeller, which wears bearings and impeller blades more quickly. This ALSO slows the impeller, or "un-spooling" the turbo.
I want to start my next statement by saying this:
I absolutely LOVE the sound of a BOV, especially a super-sequential. Just sexy.
Now, I'll tell you... I think DV is better. BOV dumps boost pressure to atmo. DV dumps pressure back into the intake side of the turbo. This may or may NOT allow for a quicker spool of the turbo, but what it DOES do is icrease the available air for the turbo to suck in. I like the idea of recycling the boost, instead of dumping it off.
If you could make a DV that made the same sounds as a BOV, I'd give you my firstborn.