When subword (resp. superword) minor mode is enabled, a comma 
(resp. a superscript two) is displayed in the mode line. I found 
those two characters difficult to see and meaning less.

I understand that there's not much space in the mode line but 
other minor modes are much verbose (eg. MML, Abbrev, Fill, Narrow, 
Outl, etc). Why subword and superword modes have this special 
treatment?

I suggest to use Subw and Supw for the user to have a chance to 
link the mode line string to the full minor mode.