GNB gets a AoE heavy that can ruin an entire retreating party's day if timed right + decent single target burst.

DRK gets its survivability spread out over time, and can seriously hurt a player's chance at getting healed back up.

Granted, I wouldn't say these abilities (esp. DRK's) are suited for Frontlines when all you're truly aiming to do is grab/bind/mark vulnerable targets, disrupt claimers, and not die yourself.