BST in party only works this way, thanks to the supportjob design. This is intended by SE, and not BST player's fault.
Can also do other setups, it doesnt matter much if you fight a T1 Escha NM with BST x 3, or a setup which consists of PLD/RDM/DD.
The second setup has one problem: Each member has to be good. The BST setup needs only one good player, which makes it very PUG friendly.