Question: A DM is following the sacrifice rules as written from the Book of Vile Darkness, and one PC in the DM's campaign is a truenamer like the one described above. That truenamer performs a sacrifice each day on behalf of the low-level party. How much will the party benefit get from this sacrifice?
Answer: Obviously, there're unfathomable benefits to a group of level 3 PCs having access to an 18 Hit Dice outsider for even an hour every day. Even if the DM sends only an ally as simple as a fiendish tyrannosaurus (Monster Manual 107–8 and 61–2, respectively) (which I know is not an outsider and merely a creature that possesses the subtype extraplanar, but give the DM a break here, okay?), it's not like the level 3 PCs will complain about having the assistance of that CR 10 critter! And were the DM to pick an actual outsider—with its host of spell-like and supernatural abilities—things would only get better for the PCs.
So, yes, the 8th-level cleric spell greater planar ally [conj] (PH 261) is unbalanced in the hands of a level 3 character. Getting a outsider servant that possesses 18 Hit Dice from one's dark master for even one hour is a boon well beyond the power of most level 3 characters. Doing so daily means the PCs will indirectly—by the dark virtue of their unholy master, unhallowed be its name—overcome challenges far beyond their levels.
However, the sacrifice rules don't exist in a vacuum. They aren't just two absolute tables and done. The Book of Vile Darkness on Sacrifice, in part, says
Most evil gods and fiends demand sacrifice. When they say sacrifice, they mean the sacrifice of a living, intelligent creature. Commonly, the victim is a humanoid, but dark powers might demand the sacrifice of anything from a medusa to a giant to a beholder. The main criteria are that the creature be alive and have an intelligence score of 3 or higher. (29)
This means, using the rules as written, the god to whom the illumian truenamer is offering sacrifices can opt to—at the god's leisure—raise the stakes. Essentially, the god says, "That kobold mushroom farmer was fine last time, but, buddy, I lent you the services of Mutae the Chomptastic, a fiendish T-rex! Another kobold commoner 1 just ain't gonna cut it!" Further, the text says
The sacrifice must be made in a ritualized manner. It’s fine for a blackguard to say that he is dedicating every foe he slays to Vecna. But in truth, Vecna appreciates and rewards his followers for victims killed in a ceremony and at a place dedicated to him. This means that the ritual takes time, and it probably involves extra participants and unholy trappings. (ibid.)
Here again, the DM is entitled to raise the stakes. Table 2–1: Typical Sacrifice Elements, does, in fact, imply that the PC can take a subdued kobold commoner 1 into a filthy back alley, coup de grace the poor dude, call that a sacrifice, and be gifted for 1 hour with the services of Mutae the Chomptastic, but the text also says those are only the typical elements, and the DM is given explicit permission to change those elements, and a DM may—capriciously, like any dark god—change them whenever he wants. (Although this longtime RPG player and GM normally frowns grimly at such arbitrariness, here it seems acceptable that a dark god become increasingly demanding of even his most knowledgeable and reliable supplicants!) Finally, the text says
The celebrant can choose what reward is sought and prayed for, but the deity might grant another reward, especially if the Knowledge (religion) check result is significantly higher than the DC of the reward sought. (27)
In other words, the PC picks, but the dark power grants, and what that dark power grants is entirely up to the dark power. Backing up a little:
Each evil deity has his or her own predilections and preferences for sacrifices, and different deities give different rewards to their followers. The… system of sacrifices and rewards [presented here] is just a starting point; [DMs,] give each evil power in your campaign its own unique sacrifices and rewards. (26)
And that's kind of the crux of all of this. When that illumian truenamer 3 whips out his ceremonial dagger in that back alley to do in that kobold commoner 1 in the name of his dark god, he's instigated a series of events that the DM is supposed to have prepared for that is unique to that DM's campaign and based on the deity to whom the PC is make the sacrifice. While it's possible that the outcome will be the effects of a greater planar binding ally spell, that's not guaranteed. In fact, the only real guarantee here is the damnation of the illumian truenamer and his fellow PCs that benefit from his frequent sacrifices.