Heh, someone else answered this, I just hadn't read far enough down. Basically, the old design was streamlined to allow for unimpeded water flow and it spread the blow over a wide area to smash concussively, rather than merely pierce. Since the old ships didn't use nails but rather mortise and tenon joinery, a smashing blow rendered much more damage, kinda like a hollow point bullet's expansion creates a wider wound cavity than just piercing straight through would. Or think a hunting arrow vs a field point.

EDIT: Your brittle comment was addressed as well- the rams were kept as short as possible.