Not all of these spells are nearly as bad as you say, and have variable uses depending on the situation.

Blood Drain is almost essential in some savage fights for tanks and occasionally healers, given just how demanding damage can be and a heavy leaning on quick heals / Diamondback uses.

Bad Breath, again, is an essential mitigation against some of the larger tank busters or AoEs - Alexander's 2x Mega Holy in A12S, for example, can be tolerated with Gobskin+full health but boy do you need to heal quickly through the back-to-back major damage, and Bad Breath eases the healing burden notably.

Shock Strike is already pretty great at only a 60s cooldown and fairly high potency. I don't think it needs improvements.

Black/White Knight's Tour used to be the go-to for many dungeon runs that didn't end in X5 or X0. A solid linear AoE to murder packs of mobs quickly. Ultravibration made that moot, admittedly, but when you can just power through adds much more quickly because of instant death, it does save a lot of run time.

Cactguard is again, another fantastic mitigation, this time for your off-tank to use in order to reduce a chunk of tank buster damage on our squishy, low-HP tanks. Even if they're coming as a non-tank role because you don't need an OT, it's still an extra bit of mitigation for when you're trying to lean a bit more on Chelonian Gate/Dragon Force for defenses as opposed to burning MP on Diamondback. This one is a bit more niche, but boy has it helped in things like T13 where Bahamut is just slapping you with tank busters and instant autos rather rapidly.

Rose of Destruction is fine as it is, and it competing with Chelonian means nothing if you're not a tank.

While I get your goal here, remember that BLU is a job with more than 100 abilities and only 20 slots to fill in. There will always be "superior" or "better" spells, and then those that are more niche. Communities will always, 100% gravitate towards the best setups for the situation, and the only way to make "every ability applicable" would be to make them all relatively identical, at which point all you're doing is reskinning the same ability over and over.