> One letter variables are supposed to be used in scopes that fit on the screen completely.

Exactly, and ideally in less space than that. If you have something like:

  for (i=0; i<10; i++) {
      foo(i);
      bar(i);
  }

There is no point in using a "descriptive" name for the index. It's completely obvious what's going on. Anything more verbose would just hamper readability.