It didn't go to waste entirely. There were a few events related to it that are still going on (like Farren's death... wouldn't have happened if not for the MB), and as I said above, if someone can come up with a similar yet different curse, the storyline could be rewritten to not include vampires, and the Monstrumis line could rise again in a different form.
EDIT: Since nobody else seems to want to do this... I ended up coming up with my own idea. I hope somebody bothers to read it.
Basically, the idea is that there was this mage who wanted to find a way to optimize the balance of the different magical energies within people. Kind of a feng shui sort of thing, only with the ways of magic. He hoped that he'd be able to use it to cure diseases and other problems people had that couldn't be cured by the Crystal Way alone. He eventually succeeded, and was about to announce the findings to the world, when one of his assistants suddenly went insane. The assistant killed the mage and stole his research, and eventually started to use what he'd learned to create imbalances in people's energies. These imbalances cause those affected to "leak" energy of one kind, and become vulnerable to another kind that opposes it. In the case of the Monstrumis Bloodline, they leak Dark Way energy, and are destroyed upon contact with the Crystal Way.
Because of the leaking energy, the cursed people must drain the same kind of energy from other people. This can be done through any kind of prolonged physical contact. However, the assistant (and those he gave the secret to) always lied to those he cursed. He didn't tell them about the energy imbalance at all. Instead, he told them that they had been "turned", and that the only way they could survive from now on is to drink the blood of the non-cursed, which did work but only because it was extremely hard to shake off someone who's biting your neck. The reason he did this was to keep control over the cursed people. He could always threaten to keep them away from blood. If they realized that blood was not needed, they could simply survive by tapping into the energy leaked by the others, band together and force the one who cursed them to give them the cure.
So, Cruentus Monstrumis was actually just one of the people the mage's assistant gave the secret of the energy imbalance to. He used this to gather an army that would do his bidding. (The whole mind control thing is totally separate from the energy imbalance curse. It just helps.)
Oh, and as a side note... blood on its own also can be drained, which is why rat's blood worked on Duvare even though the bloodsucking itself is superfluous.
