The case for the /usr merge

Just having /usr mounted read-only isn't that big a win, but if the system will work with /usr mounted read-only, it will also work (which real benefits) if /usr is storage the system is technically incapable of modifying. (That is, where changing it requires doing something entirely different, like swapping CDs or flipping a physical switch on the device or something like that; or where it is a different system which can modify it.)