I understand but why dont people like jobs like SMN or BST or WHM (apparently)? The first two do great damage, SMN also heals buffs and everything else. How is that job NOT useful? Dont get me started with WHM
The problems with SMN and BST are that, in this higher level content (Delve), neither really offer enough to warrant their spots. A Smn's damage isn't as impressive when you consider the 45 second recast on Rage, their status ailments don't do enough to make up for the spot they're occupying, and their healing isn't up to par with what needs covered.
Beastmaster is in a similar position as far as not being a very great DD. Right now Jug pets in there just aren't going to have the offensive or defensive power to stand up to a lot of these NMs, and a Beastmaster is only going to be a mediocre DD at best. However, there are some current strategies for Muyingwa (the Bztavian Delve boss of Ceizak) that do involve bringing a Beastmaster and the Lizard jug pet for Infrasonics, since it apparently is a 25% evasion down effect on it that lands at least somewhat reliably.
I've never seen anyone have any issues when it comes to White Mages. If anything, that's probably one of the most sought after jobs when it comes to anything Delve related.