I use the midline feature of blocks all the time, on the ground for winching and mechanical advantage. I'm not much of a removal climber, but I virtually always have the rigging line end when roping stuff in tree.
Rings are strong, light, cheaper, don't mind getting dropped out of the tree... blocks are nice too but I can't figure where the midline feature is a big advantage in tree. Granted im dealing with 100 footers max, maybe it's more important out west?