>> How highlighting the replacement submatches will help the user to decide
>> whether to replace the current match or not?
>
> Whether to replace a given match is not the only thing
> a user can do - not the only thing to decide/consider.
>
> Highlighting groups helps you see _how_ a regexp matches.
>
> And you can always change a regexp and search/replace
> again, if it's either not matching something you want
> to match or matching too many things you don't want to
> match.
>
> It's helpful in pretty much any context where regexps
> are matched against text, IMO.

I think you are right.  So here is a patch that implements this: