Simply because of the armor sometimes. DRG AF is appealing strangely to many people because its purple even though i am reminded of the Jade look Kain has in FFIV. I know a friend of mine wanted to be a DRG just by looking at the AF Armor and it appealed to him. As for DRKs, they hold very badass weapons, a Great Sword is like a Buster Sword, and a Scythe is like the symbol of Death, it is very badass lets just say. And for doing lots of damage easily multiplies the badass factor even more.

Although personally DRG appeals to be because i can Jump/Super Jump and to a lesser degree have a pet that will follow me and be my companion.

IMO theres nothing wrong with these jobs, in the end there will always be sucky soso jobs. I like WHM because its a healer and a pivotable point in a party in any FF(although in FFXi its less so..makes me kinda meh) and i like to help, similarily to PLD.