Both RoP:TI and RoP:F refer to demons being able to corrupt faeries, but neither book really says how demons corrupt faeries — or why for that matter.
If a demon is powerful enough to warp a faerie into another demon, then what benefit would the original demon get out of this? The faerie has no soul that would earn the corrupting demon evil-kudos in Hell like corrupting a human would. But if the demon's goal is corrupting a human who is close with a faerie, then why corrupt the faerie when it can just impersonate the faerie to worm its way into the human's ear? Not like the human would ever know the difference.
And if a demon does wish to corrupt a faerie, how would the demon accomplish this? Would the demon just corrupt a faerie forcefully, using infernal magic that rewrites the faerie's glamour? Or would a demon try to corrupt a faerie in a similar way it would a human — through deceitful dialog that naturally persuades the faerie to follow an evil path? I can't imagine demons being very successful in persuading a faerie to change its role since only humans have the Vitality that fuels a faerie's glamour and thus role-changing capabilities.
faerie's have a better interaction chart with both magic and divine, plus if someone can check realm alignment and finds it is a faerie they are dealing with their guard may be down- demons can hide their infernal natures but nothing indicates they can emulate a faerie. Faerie servants may also be more reliable than other demons as well. Additionally there is the question of whether intent is required to corrupt a faerie- it may simply be a byproduct of their interaction- a demon charges a rent in souls for a faerie to work in their aura and over time the soul stealing faerie's story changes until they become actually infernal.
probably the most obvious reason is that many faeries exist in roles where humans venerate them. Examples include friendly spirits that must be appeased, house-gods, ancestor spirits, place spirits, minor local gods, etc.
These faeries that subsist on human veneration would be prime targets for infernal recruitment/usurpation because a demon in such a position would be able to slowly twist the venerations such that they corrupt those who venerate.
But why gamble on a corrupted faerie the demon can only influence when the demon can kill the faerie, impersonate it, and get the job done itself? Because demons are straight up immune to Intellego magic, it is difficult to uncover one. Discovering the demon's true nature would most likely go unchecked without Scooby Doo and the magi/saints gang stumbling upon it by mere chance.
If we assume that the demon is able to: kill the faerie in question and impersonate it, then indeed there would be little reason for demons to corrupt faeries - sufficiently little reason that I am not able to come up with any at least.
That however is something of an assumption. I do not doubt that for specific demon vs. faerie combinations this would be true. But I very much doubt that it is universally true.
Here are my objections:
For a demon to kill a faerie it has to first find and then defeat said faerie. There is really no guarantee that a demon can find a faerie before the faerie realizes it is being chased and can kick up a fuzz about it. Remember the demon not only has to kill the faerie it has to do so before the faeries followers can become aware of it, otherwise there is significantly less value in impersonating the faerie.
Then assuming that the demon finds the faerie it has to actually kill the faerie, which is also a lot easier said than done. Many faeries are tough and can put up quite a fight. Again remember if the faerie puts up so much of a fight that the mundane followers of the faerie notice then there is much less use for the demon in killing and impersonating the faerie.
Then the demon has to be able to convincingly impersonate the faerie. Sure immunity to intellego is a boon when evading hermetic magi but if the demon is terrible at whatever the faerie did then it will still fail to impersonate the faerie.
Then there is the issue of supernatural detection. Yes immunity to detection by Intellego is an advantage but I would argue that the primary danger of supernatural detection for demons is more likely to be people with the virtue "Sense holiness and unholiness", churchmen in particular, and this virtue will not be confused by a demons immunity to intellego. After all Hermetic magi do not systematically hunt demons, while churchmen do systematically hunt, well not demons in particular but non-divine aligned supernatural beings, in particular those that are interested in humans. Thus churchmen are more likely to go looking for demonic corruption among otherwise quaint folk beliefs.
All in all I do think that there are demons with the powers of detection to find a faerie, the powers of violence to kill a faerie and the wits to impersonate a faerie but I also believe that this demon is much more likely to be 3 different demons or is that is sufficiently outclasses the relevant faerie that the faerie is beneath its notice.
In more concrete terms imagine a might 30 demon, sure this demon might be able to vaporize say a might 5 faerie that poses as an ancestor spirit for a peasant family and take its place to slowly corrupt said peasant family, but a might 30 demon has better things to do than corrupting just 1 peasant family. A might 5 demon on the other hand might be content with just 1 family of peasants but it is unlikely to be able to find and replace the faerie elegantly and to remain hidden from local church authorities.
Also there is a matter of tactics- sure the demon can kill and impersonate 1 faerie, or it can dominate and corrupt 10...
There are certain things demons cannot do but would like done, while faeries excel in them - like giving birth to a corrupt pope.
A small campaign around this is in sub rosa #16 1050 AD: Strange Alliances.
I believe demons technically could "give birth" (to the degree that works from ROP:I) to a corrupt pope, it's just far more difficult.
Basically it a holdover from The Celts Were Right in earlier editions. There's a tiny area in lowland Scotland where it is believed faeries pay souls to Hell as a sort of rent. This was made universal, with Infernal faeries all over the place. It was kept for backward comparability.
I agree with much that's been said by other responders; I would add just one point.
Demons are different from each other. In particular, they differ a lot in terms of a) domain and b) modus operandi. And these two aspects condition their behaviour much more than considerations of efficiency (unless efficiency is somehow involved in either a) or b)).
Suppose a benevolent, local nature goddess -- a faerie -- grants the inhabitants of a small hamlet health and prosperity in exchange for ... uh, the sacrifice of the first lamb and the first bushel of rye of the year. And a demon sees the hamlet, and sees the happy souls in it, and thinks "Yummy! Mine!".
Now, if the demon is a crude, brutish demon of pestilence, it won't try the corruption tack. It might uhm .. initiate some sort of disease related-mayhem, attacking the health of the land head on. Its line will be "Your herds are sick and dying, your crops are sick and dying, your goddess is sick and dying! You can't fight pestilence, you can only bow before it! Despair and tremble and worship me!". This may, or may not work in the end, but even if it's clear from the beginning that it will not work, this demon will try anyways: as a creature of pride a demon can't imagine itself failing until confronted with failure. Of course, faeries always love good stories; so even if the goddess outclasses the demon, she will have her followers think it's a really really close call. Or perhaps she will be vanquished in blood and pestilence, until some hero is born to the land ...
Instead, if the demon is a subtle demon of corruption... then said demon will try to corrupt the faerie and everything else it touches. But not out of considerations of efficiency, just out of its intrinsic nature! The demon might impersonate a beautiful foreign woman that a man in the hamlet takes to wife. And she might whisper to his ear that the reason he is siring no sons onto her is that the local goddess wants to test his faith -- she'll give him the fairest of heirs if he is willing to sacrifice a little more than the boring yearly lamb-and-rye... Eventually, this will result in the faerie being corrupted by human creativity, or some other faerie taking her place, or some demon taking her place (perhaps so that she can take the role of the "prophet of the old ways...").