Protection paladins and blood death knights simply bring more utilities to the table: divine sacrifice, ardent defender (the best passive skill of the entire game), greater blessing of sanctuary (even if is overrun by the discipline priests), hand of freedom, hand of salvation, hand of sacrifice, hand of protection, hysteria, abomination's might (which may be overrun by hunter's aura), icy talons (increasing melee haste). Just by replacing the blood death knight with a feral druid, you would need to replace a fury warrior with a frost death knight or an enhancement shaman to ensure the melee haste buff. On a realm where everything is min - maxed, you don't want that.