Without a multi-user frame sync, this prohibits users from being near-angles from each other to the retro-reflector, which basically prevents someone from "pointing" in AR into the others' field of view. Not necessarily a big limitation, but say you want to point out e.g. a terrain feature. If the software lets you select it, awesome! but if it's unselectable, you just have to try to describe it "the tree three trees over from the west wall... no not that one". Having good 'pointer drop' support mitigates this I suppose.
You can see the person's finger (or the CastAR wand's tip). I imagine the rendering engine will show the same thing in the same place relative to the real world for both users. It doesn't have to, in the case of a game where you each have your own point of view, but it could if the game works well that way.
