Skimming the thread, I see that bones pointed out the presence of stylized masks, but I don't think anyone's brought up how frequently they probably appeared. In set form there was only a small handful, but I remember vividly that in Bionicle Adventures #1 the narrator points out that Nuhrii had a "signature style" or something to that effect--ridges around the eyeholes or something like that. So even if masks were largely identical (that is, more identical to each other than humans' faces are) there would certainly be minor differences like these, not to mention physique differences, scars, and possibly voices.