Sanguine Rite is somewhat useful but nowhere near what it should be.

.It costs way too much mp for what it does.
.It is considered a spell although it should be a buff-type action so it actually BREAKS your spell combos if you need to cast it in the middle of them.
.Its duration is too short (if you cast it at the start of your combo and get one spell interrupted by other means, or have to move for a bit it will never last till the last spell of your combo, which where you usually need to have it active).
.The MP restored when damaged mechanic it has is flat out BAD. A DD mage that is made of paper needs to go get hit to restore a low % of the damage taken as MP. It's MP mechanic is even self-defeating since when you use it on THM/BLM it lowers your damage taken, inherently reducing the amount of MP it will restore too. You won't even get back the spells' MP cost unless you have it active when you get hit with Hellfire or a similar high damage ability.
.It's party utility is almost gone since BLM doesn't have access to Sacred Prism to grant the buff to the whole party.

What makes it useful at the moment is that it can lower damage taken, which actually makes it the only damage lowering ability Blm now has, since we can't access Sentinel.

All in all, they should consider scrapping Sanguine Rites in it's current state and consider remaking it into something along the lines of:

.Freely castable buff on self but also allies that gives immunity to spell interruptions from damage sustained (Could have it at 4 min to go along the Protect and Stoneskin buffs). Cross-class available. (This option leaves Blm with no damage reduction ability at all which may cause problems in situations like Ifrit).

.Ability (NOT spell), with no mana cost just a CD, activated on self only and prevents spell interruptions from damage while it lasts. Cross-class available.
THM/BLM Trait: On first damage from any source sustained while it is active it restores a flat amount of MP (amount rises with Class/Job level). While the buff lasts damage taken is reduced by a flat % (like the current version).

There's nothing more ANNOYING at the moment than finding yourself, being a Black Mage, in a situation where you have just enough MP left for 1 good spell that will kill a monster/put you out of danger but you KNOW if you try casting it it will just get interrupted. Sanguine Rites to avoid interruption? TOO BAD, not enough MP to cast the spell now. Same shit as trying to finish your Fire combo to AoE that group down but you know if you even think about casting Firaga you will just get interrupted and by the time you attempt to cast it again combo timer is over. Sanguine Rites will fix the interruption but will also break the combo (DERP).