So let's accept the statement 'all cultures are equal' at face value: all cultures are equal and therefore there exists no metric for measuring 'goodness' or 'effectiveness.' If C(now) captures my current cultural state and C(future) captures my future cultural state and we're saying both of these states are equal - because all cultures are equal - then where's the impetus for any new legislation? Where's the drive for change?
In the United States our past culture included slavery, which we'll call C(slavery). Are we really saying C(slavery) == C(now)? I bet most people would claim C(now) > C(slavery) and if so should be suspect of any claim that all cultures are equal.
This isn't a rigorous proof, but I do think it calls into question the notion that all cultures are equal. But maybe that's not what the phrase actually means: maybe what they really intend to say is we've yet to identify unbiased metrics by which we can use to measure cultures and thus correlate them. Presumably most people would bias the metric in such a manner that their culture turned out to be the best. If that's what's actually meant by the phrase 'all cultures are equal' (and I've never heard of anyone presenting it that way) then it should be stated as such because then we can work toward identifying those metrics so we can objectively improve our own culture.