The biggest advantage for 5 dks is that their lvl start at 55. Don't think they have major advantages over other melees at level 80. With that said, they sure can be very fun. But When it comes to PVP, they need some major buffs to be competitive. I did some testing with 4 DKs at lvl 80. A Holy paladin friend who is fully pvp geared just stands there and let me beat on him. Took me like 1min and 20 seconds to bring him from full - 0 hp. I only interrupt the Holy Light. Let the flash of light go through.

The ramp up time on damage is just too long. Without full disease stacking, you have no burst. and the kiting is also a problem. The death grip only grab your targets to you. It doesn't make you move faster. In BG, mobility is everything.