If you go BLM/SMN the reason why you sub Summoner will NOT be because of their Devastating powers but for their Huge MP supply which will benefit your BLM, this combination will make you able to level up in Parties only and Not by soloing.
BLM/BST on the other hand have very few advantages.
You will Not be invited to parties because the BST sub will Not benefit your BLM in anyway.
Only thing you could do is to Charm some random Enemies to help you because of the High Charisma you gain from being Tarutaru (CHR=Charisma and it helps Beastmasters to charm=tame enemies).
Alså the BLM main job will not help anything because if you attack the enemy he Will attack YOU instead of your Pet which will result in instant death if you want to fight something with good exp.
In other words if you want to play as BST DON'T sub it because it's just a bad choice. play BST/WHM or any melee job as sub.
BLM/SMN is a great combination but DON'T think the avatars=summons will help your blm though.
Hope this made any sens! If you want BSTs of destruction choose SMN or BST as a main job if you want magics of destruction choose BLM as main!
Btw no one said you cannot have them all
Have fun and good lucl!