The only problem i see with the Elemental Staves is that the jobs that use those, Generally don't tend to Melee, ever.
only exception i would imagine would be COR and RNG, but COR Actually gains damage on most enemies by using Fire/Chatoyant, and RNG has had replacement for the Fire/Vulcan for a while i believe. (Outside of Wildfire).
Thief's knife is worthless for DD, not whats its used for. I carry one in my mog sack for those times i might be fighting something like Fafnir where I care more about highest TH Than i do Killing it quickly, since the goal is the drop. (This is different for time based events like Dynamis).
Still your reasoning is pretty much how i saw it too. They wont give us Subtle Blow because we upgrade TH on attacks, and if we got Subtle blow, Banana Purple Lake Monster. Or something, It made no sense and i hate Square enix every time they open their flaps about Thief, as it makes me want to drink acid to wash down the gurgling idio......... Calm down karby...
But yah, Its probably got something to do with BALANCE.
I mean hey, the jobs already Balanced into a steaming pile of mediocrity, lets tack on a sh*tty level 70 weapon to really twist the knife into their backs.
Get it? TWIST THEIR KNIFE INTO THEIR BACKS? ITS A PUN YOU SEE, CAUSE ITS A THIEFS KNIFE