The cap on the limb tips is there simply to reinforce the tips of the limbs and prevent any splitting there. They designed the cams so that they don't need to use the cheap plastic spacers that so many manufacturers use. The sealed bearings take the place of the spacers that others use.

The spacers is a way to cut manufacturing costs becasue a little spacer is much cheaper to produce than the extra aluminum that must be machined off from the billet so that the spacers aren't needed. Also the tollerences can be held much tighter machining aluminum with sealed bearings than a molded plastic spacer.

The spacer method is not the superior system. Think if there was a plastic spacer of some kind on the wheels of your truck.