NIN/DNC can solo their own Kannagi. Must be over-powered! As can DRG/Mage if they know what they are doing!
However, the point is: Sure BST can solo just about anything. A 'naked BST that didn't even bother to learn about the job' can't. A BST that can solo just about anything has a ful pet -PDT set, -MDT set, Pet DD set and (wait for it...) yes a BST that wants to solo these really hard mobs will actually have a melee set. This is because a decent BST will actually be meleeing along side their pet, or on a mob with really dangerous AOE just running in to WS, and using snarl. A decent BST will have an idle set with a huge amount of auto-regen.
A naked BST isn't getting the most out of their reward.
A naked BST is going to take (even) longer to kill stuff.
A naked BST sucks.
A naked BST will often be seen sampling what the floor tastes like.
Sure a BST can solo most stuff, but it's not as easy as the OP makes it out to be, and what's more, it can take forever to do so.
A poor BST in our LS pretty much solo'd his own Masamune without asking anyone for help. He spent a lot of time online, and it still took him months. We only helped when we noticed we were in the same zone as him for whatever reason and spammed a few pops for him, or helped him out on procs.
Party of the right proc jobs/well-geared people is and always will be > BST solo.
BST solo is fine if you are in the (un?)fortunate position to be able to play more often than most of your friends, so you are online by yourself a lot, or just if you have no friends; but you're much better off asking friends to go along, let them lot whatever you don't need and get it out of the way as fast as you can.