I personally use read only files a lot, it's nice to also access them from Wine file explorers/managers. Of course, Wine can't set read-only on directories, because Windows can't (at least not with same semantics as Unix), so that's fine.
You mean files on the Unix side which you've manually set as read-only via chmod? I don't think we'd be breaking that case, it'd only be if a program tries to open those files with FILE_WRITE_ATTRIBUTES, which should be relatively rare. The only known example is Cygwin's unlink(), which is going to need write access to delete the file anyway.