I often use an x-ring in the lower third of the tree, to improve rope angles and add extra friction in natural crotch rigging. Nothing to retrieve up top, the low ring is easy to retrieve before coming down.
I love that old video with McMahon. Beranek also covers this technique in The Working Climber. This is great when just straight down-rigging, you always have access to adjust the rope as needed at the stub.
If I'm rigging from a remote overhead position, this is not available to me. If my rigging point is 30 ft above and 15 ft to the side of the horizontal limb I'm taking out, I'm not going to take the chance of a round turn on a crotch up top. For one, it ruins my chances of any decent amount of tension in the line before the cut, and two, if it ends up being too much friction then I'm stuck.
I've had situations where I'm trimming a tree on a fence line, where the trunk is on the side opposite to the work coming down. If I'm doing some heavier stuff, I have no place for a bollard, without taking my ground guy and placing him on the wrong side of the fence. This puts him out of the mix for landing pieces, dragging brush, and helping out in general. With a small crew this can be a real problem. I usually just go natural crotch anyway, and we can handle most things this way, but there are times where the pieces are just going to be the size they are, not always much option to cut them smaller. In these cases, which are rare, an aerial friction device can be the ticket. I use my homemade thing maybe 3-4 times a year, which annoys me because I haul it around to every. single. job. all. year. long.
This is not directed at anyone in particular, but if you can't think of a single situation where an aerial friction device would make sense, then you're not using your imagination...
I am still hoping someone can chime in on the pre-tensioning ability of the DownRigger. It's way too expensive for me to even consider it, but I'm interested to know...