Are all souls of equal value?

Question

The Monster Manual's Devil entry describes how devils desire souls and enter into contracts with mortals to gain them.

To own a creature's soul is to have absolute control over that creature, and most devils accept no other currency in exchange for the fiendish power and boons they can provide

However, it seems to make little difference what the creature used to be in life, a common powerless beggar, or a noble, high-level paladin. They all just turn into lemures:

When the soul of an evil mortal sinks into the Nine Hells, it takes on the physical form of a wretched lemure.

As this question asks tangentially (the main question is about ways to create magical pacts): why would a devil even bother to try and seduce an archmage or high level paladin, which is a lot of work as these victims can be resourceful, smart, wealthy, hard to seduce, and therefore costly and difficult to win. On the other hand, the devil could just offer modest riches to a beggar, who might be a lot more susceptible to to escape their wretched living conditions now and trade their soul for it. That seems like a lot less work and risk, and in the end, the devil gets a soul-turned-lemure either way.

Is there something inherent in a soul of a powerful, high level creature that makes it more valuable to devils and other traffickers in souls (night hags, for example), than a poor commoner's soul, and justifies the extra effort?

Answer 1

Devils value a powerful creature's soul higher a than weak one's

In the core rules the only hints that a more virtuous soul is of higher value is found in the Monster Manual, in the lore entry for the Succubus/Incubus, and that is pretty oblique:

The more virtuous the fiend's prey, the longer the corruption takes, but the more rewarding the downfall. After successfully corrupting a victim, the succubus or incubus kills it, and the tainted soul descends into the Lower Planes.

However, Mordenkainen's Tome of Foes describes the infernal hierarchy in more detail, and offers the following (p. 18):

Each time a devil signs a contract that pledges a mortal's soul to the Nine Hells, that devil receives credit for the achievement. A stronger soul, such as a mighty warrior who leaves mortality behind to become an ice devil, is worth more than a simple peasant likely to be consigned to existence as a lemure. [emphasis added]

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In addition, powerful souls can have effects on the Abyss (with whose inhabitants, the demons, devils are constantly at war (p.26)

A powerful soul might be able to dominate demons, retrieve weapons of the Blood War to use in mortal conflicts, or discover spells known only in the Abyss [...]

The Archdevil Dispater is interested in souls that know secrets (p. 11):

He covets the souls of those who seek secrets and those who have useful, secret information of their own that he can bargain for.

The description of Mammon also makes it clear that different souls have different value (p. 12):

Devils that are tasked with harvesting souls for Mammon carry with them The Accounting and Valuation of All Things, a manual that guides them in assessing the value of a soul in gold or other goods.

Baalzebul's entry points out that the souls of unskilled, weak creatures count for little in the infernal reckoning (p. 16):

Although Baalzebul claims a great number of unsuspecting souls, almost all of them are pathetic, incompetent wretches best suited for the dreg legions, whose only task is to die as slowly as possible so that they might delay the advance of an abyssal army.

And lastly, Mephistopheles (p. 16)

harvests the souls of skilled wizards and cunning sages, exactly the sort of folk he needs to further his research.

All of this supports the idea that the knowlege and prowess of a creature in life is somehow retained or transferred to its soul, and will influence how competent it will be to become more powerful in hell. Therefore, the souls of more powerful creatures are valued much higher in hell. From a devils evil perspective, beggars may be cheap to pick up, but you get what you pay for.

Answer 2

"When the traitor's hand strikes, it strikes with the might of a legion."

If a fallen paladin is a member of an order of paladins or some other group that actively works for patrons from the Higher Planes, and his perfidy is not yet known, he can be of great service to his new patron while he is still alive. He can gather intel, sow seeds of doubt, turn coat at a critical time of a battle, etc. This depends on how well he is able to maintain the charade.

Remember also that because the powers of the Lower Planes hate each other every bit as much as they hate the powers of the Higher Planes, a former good-guy can simply steer his comrades to focus their efforts against one of the diabolical patron's rivals. Not only does this serve the interests of his new master, by openly opposing a known force of the Lower Planes, his true allegiance can be more easily hidden.

Even if the charade is pierced, he can still go around as a fallen paladin, sowing evil by less insidious means, obstructing and defeating the mortal followers of the Upper Planes wherever and whenever possible.

These betrayals, on top of advancing the patron's schemes, will also have the effect of further securing the fallen paladin's fallen state, minimizing the chance that a deathbed repentance (curse those all-forgiving Upper Plane types!) will rob the patron of his lemure.

This dastardly career will go on until the patron devil decides that the fallen paladin is no longer useful. Then he will withdraw his protection and arrange for his death; and because most higher devils have a sense of humor, it will come at a point when the fallen paladin has come to believe that this will not happen.

The same goes for any other person of high status who is turned to the path of evil.

Given their preference for organization and well-planned schemes, this is precisely what we can expect devils to do with a new recruit. Yes, if the diabolical patron has him killed today, he gets a new lemure today, but if he can play the long game he gets a period of service, and then gets the lemure.

As for beggars, they are probably easier to drag down, but if they were useful for more than cannon fodder, they wouldn't be beggars.

(If you have the opportunity, read the speech at the end of The Screwtape Letters. There the author makes the point that people who aren't very useful to Heaven typically aren't very useful to Hell, either.)