If you play a 5 string tuned BEADG and your guitarist is tuned down to DADGBE and you are playing riffs in unison, the D on your b-string will not sound as powerful as an E string dropped to D and played open. You will also lose symmetry with the guitar player whenever hitting the D.
For any type of music where the guitar is dropped to D and you are playing parts in unison it IS necessary. The open string chugging is a big part of the metal sound.
So, um, why not just tune a four string DGCF? That's what I do. Open D. I have no use for Drop D except covering Tool or doing similar things.

