Sure Manyakumi, I know that. Even if you were trying deliberately to offend me I would enjoy it. If not for your input here this forum would be BORING! You are one of the most lively participants here! And I just loves ya more than you know.
Now to the grammar:
Yes -- Just like thousands of other examples what we learn that is grammatically correct may not necessarily be used in common conversation. Perhaps every student of English learns "How do you do? It's nice to make your acquaintance." But almost no one in the English speaking world uses that exact greeting anymore, and in everyday conversation it would be more common to hear something like "How ya doin? Nice to meet ya."
But getting back to the use of 겠 for making an expression "more polite" -- well I see something similar in English when using a future tense of certain terms, as in the case of "I don't know" and "I wouldn't know" these have no actual difference in meaning in common conversation, but "I wouldn't know" can carry a little softer connotation, and even sound more educated; however, depending on intonation and context it can also be sarcastic, or even carry the meaning "why are you asking ME?"
Once again getting back to the "polite" 겠 verses the 'tense' 겠, the interesting thing I find is that in many cases of 겠 we can still see a future sense to the expression 예: 뭐뭐 주시
겠어요? 아직 그 '뭐뭣'을 안 받았거나 가지고 있지 않으니까 그 간단한 표현에는 미래를 가리키는 뜻도 보일 수 있는 것 같아요 - 특히 서양사람의 생각(의식구조)으로는요. 또 "모르겠다"할 때는 '아직'을 말하지 않아도 "겠" 있으니까 "I don't know YET" 라는 말이 쉬게 떠오르는데요. .. 물론 나 같은 서양 머리속에서 말하는 거에요.
Following that, perhaps 모르겠다 is more polite than 몰라 because of the use of 겠 ... I wouldn't know 난 모르겠다 모르겠어 모르겠단 말이야 몇 번 말해야 알겠니? ...
of course we would all have to agree that the use of 겠 4 times in the previous sentence can do nothing to make the sentence polite - right?