why do hardware designers think that all four edges of a PCB are
fair game for off board connectors?
it makes mechanical design a lot more complicated than it should be.
True and a good point - the best alternative is the fpc connector and they are both cheaper and denser.
Companies like Itead make flexible PCB prototypes - easy to custom build fpc ribbon as needed. http://imall.iteadstudio.com/open-pcb/pcb-prototyping.html