... If they were applied to all General Delivery mail you'd think they'd be more common ...
Agreed. I estimate that only a small-single-digit percent of the General Delivery mail I've seen has a [_TAKE_OUT_]
Moreover, if memory serves, I've pretty much only seen that marking on GD mail at larger post offices.
is that smaller post offices did not bother because they pretty much kept all of the GD mail in one stack and, when anyone dropped-by to inquire, they just fanned thru that stack and pulled-out that person's mail and, while they were at it, tossed aside any really old pieces they spotted.
The larger post offices got enough GD mail that they needed to sort it alphabetically ... at least, say, grouped by the first letter of the last name, maybe more coarsely (A-D, E-H) and maybe more finely (A-Am, An-Az) as circumstances might require.
With that larger volume, it made more sense to prominently date the pieces, and keep the thinking to a minimum while doing the TAKE OUTs for the whole section once per day/week/whatever.