I have read this thread with interest, and as a competitive racer of some years, agree totally with what has been advised, but would like to add another consideration.
I would not call myself a modeller, as the building of my boats is done in the least possible time so that I can get them on the water.
I know that there are many, who will take the time (and have the skill) to make a superb model which not only looks good but functions well. I do not envy them this skill, as I enjoy the racing part much more than the building, but that's just me!
There are also a large amount of the general public who are not even aware of the different model boats and how you can use them, so we should not sneer at those who buy RTR boats and then use them as playthings - this is what they are - toys!
I know the argument that they will most likely break and the person will be put off racing boats for life, but this will happen anyway with many different types of 'impulse buy'.
Now, if that person has joined a local club or a racing society, they will most likely be offered help to get the thing running (perhaps more slowly than some of us are used to) to the joy and gratitude of the owner.
Perhaps from this beginning, the person will watch the faster boats with envy, and then want to 'improve' their hobby. This is when they will see the advantages of building their own boat - after they have gained a little knowledge about what is wrong with their current pride and joy! You could say that this will be wasted money, but very few begin a hobby with the very best equipment, be it golf, flying or whatever. So this at least gives them an introduction to the world of racing boats.
There will always be those who "want it now", the only input being the outlay of their money! They will often move quickly on to the next 'fad' which takes their fancy. It happens in all hobbies.
So please be nice to those who have brought their 'toys' down to the lake. Remember how most of us started - little knowledge but a lot of enthusiasm. All they need is a good example and a few friendly words. Eventually they will realise that the advice is good, and maybe take heed of it.