Just because you might want to search the current buffer
case-(in)sensitively, that in no way implies that you want to
search a set of files somewhere the same way.
`grep' is an external command. It is not about acting (e.g.,
searching) buffers within Emacs.
Ok, let me change my question.
Should `grep' family of commands in Emacs pass '-i' as default,
regardless of the `case-fold-search' value in current buffer,
if the user didn't customize `grep-command'?
That's a better question. I don't see why it should, but
it's a fair question.
The real point, I think, is that users can get their
preferred default behavior by customizing `grep-command'.
What the default value of that option is, is a secondary
consideration. What's the argument for changing to -i
for the default?