I guess it sounds to me like it might be "political" speech -- for example:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.
... also ...
You're either with us, or against us
As you said, I think that's "argumentation, a tool to demonstrate power, used also for winning another's favor".
Alternatively it's "parental" speech -- for example a parent or a teacher might say, to a misbehaving child, "No, we don't do that here" -- or use other plural forms, like, "It's time for us to wash our hands, before supper".
I think that kind of parental we, fits with the way the plural is used, in many Indo-European languages -- including in German for example where a respectful child might refer to a teacher as "Sie" rather than "Du".
A synonym is to say "one" -- "one doesn't do that". That is archaic or at least old-fashioned in spoken English -- excessively formal, it sounds frosty rather than warm -- but is current in French and commonplace, friendly and unexceptional. It seems to me there's more social cohesion and bit less class warfare (rivalry) in French than in English, perhaps due to the educational systems, so people might not mind so much hearing "we" in French.
If I may say so, when the Buddha uses "we" in MN8 it sounds to me like this -- i.e. that he is speaking as teacher and leader -- another person's saying "we do this" might sound like "my way or the highway" (i.e., in an extreme case, "obey me or be banished as an enemy"), but coming from the Buddha it is good advice which "we" welcome (choose to welcome).
I tend to prefer "I-messages" when I speak or write, partly because they're low-conflict, and partly for the other reason you gave (i.e. to avoid a "lie, as for how could one speak for another").
It's theoretically possible too to use a 3rd-person singular -- e.g. "this person", instead of "I" -- but that is unconventional in English.
And beware, that can be seen as an affectation or an eccentricity (e.g. narcissism or depersonalisation), if that matters, and unfortunately that syntax therefore irritates some people who hear or read it.
Apparently (I didn't know) there's a technical word for that, Illeism:
Illeism /ˈɪli.ɪzəm/ (from Latin ille meaning "he, that") is the act of referring to oneself in the third person instead of first person. It is sometimes used in literature as a stylistic device. In real-life usage, illeism can reflect a number of different stylistic intentions or involuntary circumstances.
I've only ever heard people call it "referring to oneself in the third person".
Incidentally Wikipedia also says ...
Psychological studies show that thinking and speaking of oneself in the third person increases wisdom and has a positive effect on one's mental state because an individual who does so is more intellectually humble, more capable of empathy and understanding the perspectives of others, and is able to distance emotionally from one's own problems.
... which I hadn't heard before.
Generally though I've used "I", because it is conventional, and I'm not trying to claim to being the spokesperson of a group. Also, not to leave you with a wrong impression since I've said this much already, I try not to judge people based on their use of English syntax -- I think that Postel's law is a good idea ("conservative" talking and "liberal" or permissive listening) -- generally I'm grateful if people produce English well enough for me to understand, I don't want to judge people by that.
"I" seems to me friendlier than "we", or "this person".
Perhaps some people might not want to seem "friendly", and would rather seem "formal" -- and that's the other use-case for a "we" in English -- i.e. the "royal" we, which I understand as a claim of political power or perhaps of social hauteur.
Or a family member, a parent, wife or husband, or child, might use "we".
If or when you want to talk about the Dhamma, with authority, it might be better to avoid personal pronouns -- for example you might say, "Compassion is good, anger is bad, the Buddha said", etc., all without referring to "we" and "I" and so on.
I think the Buddha usually talked like that, in the suttas, more often than not -- maybe that's called "objective" rather than "subjective".