it doesn't provide much extra privacy, given the rainbow tables are trivial to compute, as you say, which is why we haven't prioritised this historically. moxie wrote well about this at https://signal.org/blog/contact-discovery/.

however, it does provide some defence-in-depth against Identity Server inspecting the email & phone number details in plaintext, so we'll go sort it out as per https://github.com/matrix-org/matrix-doc/issues/2130