From a psychological point of view (and thus business point of view) people remember groups of numbers much more easily than they do letters. This is of course helped by the fact that there are fewer numbers than there are letters.
Also as pointed out in a previous post 'internationalism' of numbers is far greater than that of any given alphabet, and of course numbers are the same language '10' is '10' in English, French, Spanish, German, etc., etc., and yet although all these languages use the same alphabet words for the same object can be different in each of them.
Now NN and NNN domains are valuable because they constitute an easily recognised and remembered domain address, indeed certain numbers have an associated value because of perceived luck, etc..
NNNN domain names have really ridden on the back of the value of NN and NNN domain names, although a four digit number is readily memorable to all mature humans. The problem comes with larger numbers just the same as with long words, people do not remember them so easily.
Hope this simplified explanation helps.
Bill