I guess putting the 30kph limit makes people slow down to 50ish or below - which is probably about the appropriate speed.
If you slap a 50 sign onto it, people we'll do 80 and kill themselves...
I think that's exactly it. Problem with that though is it encourages people to look at a speed limit sign and assign their own perception of what is appropriate, and there is a lot of variation in that perception across the population. If limits were more sensibly applied, and more consistently enforced, then people (myself included) might start paying attention.