The ability to not die if an irresponsible person cuts your anchor leg of rope...but you already eluded to conceding that point. I believe it was Lawrence Schultz who had his base line knicked when working a tree with multiple climbers. Scary stuff...but again not needed for every climbWith a base tie you automatically have a retrieval leg and can do as many redirects as you please. Work the tree on the same line you ascend on.
What do you gain by adding a mid-line attachable friction saver?