Backing up rigging to a tree

[chh]

Yeah, I think this particular way of rigging is just a way to prolong the life (and length) of a skinnier rope not really to provide a "back up" though the language that is quoted from ACT does seem to suggest they consider this a backup. I have an older version and I don't think this figure is in there. In many activities, the ends of a rope tend to see the most abrasion from use. The skinnier the rope the sooner you see it it seems.
This provides one way to reduce that a little. It is more complicated than it needs to be, but I don't think dangerously so. Just depends on what the goals or circumstances are. Unnecessary for an 11mil rope. Maybe helpful for an 8. A reduction in strength yes, but as has been mentioned there's the gear to consider. Even an 8 rigged well with a knot is going to be stronger than your ascenders I would guess. But with skinnier ropes some people might say that maintaining the full strength of the rope is more important than the abrasion it might suffer from trees, hitches or knots over time. To each their own. I don't think of the anchor in the figure as redundant, as the anchor is the tree, but I also think the anchor is safe. I'd have no qualms using it.