Hi Layla,
I think you forgot one important use of quotations. In mathematics for example, you won't say:
"Here I think a^n+b^n=c^n has no solutions in the in non-zero integers a,b,c for n greater than n. And actually let me prove it to you."
You could say that, but you would likely mention that it is known as Fermat's last theorem. Otherwise you could be considered stealing. Similarly you will mention Euclid and others. Those are not your theorems but someone else proved them. Similarly in many cases here. I can't say I thought of something when in actuality I'm repeating what someone else is saying.
Moreover, regarding you point 1, if I were talking to an undergrad in math who claimed a theorem to be true, I would react differently if Gaoss would be quoted, or if he quoted himself. It doesn't mean the theorem is correct. I'm sure Gaoss made mistakes, but chances are better for it.
To point 2. That's true what you say, but besides my previous point there is a deeper issue here and that is the one of reading. It's a hard point to explain so I'll just say briefly that some people prefer to talk through the voice of others; through a sort of ventriloquism. Or more correctly through the tension of the text and your own reading of it.
I can say personally that I use that a lot even though I know it would be easier to not tell people how I got to certain ideas. To simply tell them the answer. But I feel then that what I would tell them, even if not stealing, would be a lie. That somehow extracting the origin of the thought, its source of inspiration, misses the whole point.
For example, sometimes when I argue I quote a TV series as you mention, but then I know with certain people that they won't like it as they don't see these series as having anything to do with reality. Can I just say what I learned from them - maybe, but not exactly.
As for point 3, I think people are here for many reasons. But there is again a bigger point here and that is who are you talking to? I'm talking to you now, but I'm also talking to anyone else who might read this. Moreover, I'm talking with dead people - I'm talking with Kant, and I'm talking with Montaigne. They are here with me and part of the argument, and I might quote them similarly as quoting you :-)
Your question is a big one and some of them might have thought of it. Montaigne for instance (to throw a BigGuy) didn't name the source for his quotations though he still always made it clear they were quotations. Many have debated this question and when responding here I'm also responding to them, and so it is reasonable that I would quote them.
I think in general your complaint is not so much about the people doing the quoting but about the people doing the reading. You dislike how people read BigGuys as if what they say is true, and you are completely correct about that. That is what you should be complaining about - how people read and how they accept what they are told by BigGuy without a critique.
(Then of course there are those who quote only to seem smart themselves and with no regard to what they quote. If they are quoting Kant obviously they are smart right. But here again we get into some silly psychology of how people in arguments try to convince other people they actually are "in the know" and should be listened to, no matter how stupid what they say is. Or, for instance, if they find you made a typo or a grammatical mistake obviously your whole argument is meaningless. But all these are endless psychological techniques of argumentation and are not the topic of your post).