There is another reason for multi-bladed props for full-scale. Matching the blade area to engine power. It is also a clearance consideration sometimes. However, it's primary reason is needing more blade area. The 7 blades on some turbo-props is a perfect example. Imagine the diameter of a 2 bladed prop with the same blade area. And of course, as you mentioned, imagine the scream from those 2 tips.
You can look at this another way, but it is really the same thing. A lower speed but larger diameter airstream and a higher speed but smaller airsteam on the ground may have the same thrust. But at speed the prop can only accelerate the air to the same speed and thus has less thrust at speed than the smaller but faster airstream.