Wikipedia claims (although the latter claim does not cite a source) that:

High-end keyboards that provide full n-key rollover typically do so via a PS/2 interface as the USB mode most often used by operating systems has a maximum of only six keys plus modifiers that can be pressed at the same time.[4] This hinders fast typists, ...

In what way would the system being able to recognize only six non-modifier keys at once hinder a fast typist?

I consider myself a relatively fast typist and I usually press one key, plus modifiers, at once; I can't imagine any real-life situation in which the system only recognizing six non-modifier keys being pressed at once has been a limiting factor in my keyboard usage. (Multi-stroke keyboard shortcuts as used by high-end software like Visual Studio, Emacs and the like are a different matter.)

Note that I am not really interested in answers centered around multiplayer computer games; I'm looking for answers that give reasons that would be relevant to typists, somehow supporting the statement made on Wikipedia.