This explanation is going to sound completely off subject at first. So, please, bear with me.
My wife and I were walking down the street one day, and I was walking on the inside (with her between myself and the street). She stated that this was un-gentleman-like. Her logic was that, if a car drove past and splashed a puddle, then I should be between her and the street in order to block her from the splash. I noted that her logic made sense. However, the custom is for the man to walk closer to the buildings. Do you know why?
Back in the 19th century and before, it was customary for the man to walk on the side closest to the buildings. And, the woman would walk on the side closest to the street. This custom came about for several reasons, but most of all because it was common for people to throw their wash water and even chamber pot contents out of the window and onto the street. With the man walking closer to the building, he was more likely to take the hit. A horse-and-buggy on a cobblestone road does not create the same kind of splash that an automobile on a paved road can. Therefore, the bigger threat came from poo raining from above.
Now you may be asking yourself "what does that have to do with the price of eggs in china?"... Well, I am going to tell you...
My wife's logic, though sound in today's world of automobiles and technology, did not fit with the logic of yore. Two worlds had completely separate views on the same subject due to their differences in circumstance.
JK Rowling impressed heavily the idea that there was quite a bit of cultural difference between the magical world and the muggle world. She also impressed that there was a lack of tolerance of muggles by many wizards.
There were several times throughout the series where Harry encountered items that he was not familiar with, but were common among wizards (talking portraits, Bertie Botts Every Flavor Beans, etc). In fact, I can guarantee that you never hears the word "muggle" before the Harry Potter books came out. Yet it is a common term in the wizard world.
So, it makes sense that there would be many muggle items that wizards might not be familiar with. Especially since the function of so many wizard items is not outwardly evident (if a muggle picked up a remembrall on the street, do you think they would have any clue what it was?).
Without the common understandings that we take for granted (having grown up with these things), it might be easy for a wizard to assume that there is a purpose for something beyond the obvious.
As for specifically why Arthur did not know the purpose of a rubber duck... Don't read too much into it. It was written by the author to point out how foreign the muggle world is to the wizard world - Even something as mundane as a rubber duck is a mystery to these people. But, primarily, it was a joke... a gag. Something to make the reader laugh. It is not a vital plot point. JK Rowling's whole colorful world of Harry Potter does not come crumbling down because Arthur Weasley does not know what a rubber duck is.
And, if you MUST have an explanation about the duck. Think of it this way... Perhaps he is so overly excited about muggles that he figures that ther MUST be some function to such a common item other than the obvious. Remember, he comes from the world of wizards, where the function of items is not always evident. Maybe he assumes that a rubber duck is some sort of protective talisman or guardian. As for why he could not have gotten this information himself... Where, exactly, does one GO to find out the purpose of a rubber duck? (the internet does not count. For one: wizards don't have computers. And two: when the first books were written, the internet was not quite as extensive as it is today... dial-up anyone?) Besides, if he had asked most muggles what the purpose of a rubber duck was, he probably would have gotten the same answer that he got from Harry (which was nothing, by the way)
So... short story long... I hope this answers your question
