I'm trying to think of an example where the NPC might not need to be at least potentially manipulable. Players are tricksie and sometimes zig when you might reasonably expect them to zag. Not that every NPC needs to be deep or anything, quite the opposite, but if the players unexpectedly decide to interact in a more meaningful with one of your cardboard cutouts, having at least a couple of short tags to work with is a good idea. I'm not advocating for ridiculous over-design or anything though.
I disagree. For example, in my Sigil game, one of my players has a contact as part of his background that provides side-quests. This NPC is always helpful, never duplicitous, and always is on the side of the PCs. This is true because it's a background investment by the player, so it doesn't bite in at all, much like taking a feat shouldn't bite you. This NPC is a vehicle for the player to engage his PC's goals, and I keep him free of manipulation. I have a few notes on appearance and mannerisms so it's consistent, but nothing else -- Saul the Fixer will always align to the PC's goals and be a good ally. I think it's important to have elements of the game that are always PC allied, else the game turns into Suspicion and Paranoia.
Now, another player's PC has earned a relationship with an Illithid through play, both successes and failures, that is a tenuous ally that clearly has it's own goals. That relationship is fraught and always a challenge to interact to see if you come away better or worse for the deal. As an ally, he will usually get a deal that's somewhat beneficial to him (so success is, in some way, baked in), but what he has to pay to get the deal and/or what plots the Illithid advances as part of the deal are open to negotiation.
These are very different allies, but still examples of two ways I approach allies. The first is, as noted, part of the PC's backstory, and, as such, is reliable. The second was earned through play and, as such, can be adversarial even as an ally. It might be possible to get an ally such as the former through play in my game, but unlikely. Just as unlikely would be to get an unreliable and fully adverse ally. Usually, you'll get something like the latter with multiple failures -- in this specific case, the PC is an ex-Illithid thrall trying to reconstruct their past and had made some big failures at crucial points in this quest so has 1) learned that he volunteered to be a thrall (this was consensual, or, at least, the players agreed to the threshold at which I'm able to screw with their backstories prior to play and this player crossed that threshold) and 2) while looking for allies against the Illithids, both failed and succeeded, so he found an ally, but it was a rogue Illithid with an uncertain agenda that has resources and knowledge that aid the PC. Yup, I'm beating on this PC pretty hard. He likes it.