In theory, I don't mind some of these tags existing. We have feet out of frame which are for nearly full body posts except only the feet are out of frame. But that is not how these tags are currently used. The current posts should be nuked but I don't see the problem with repopulating them to be like feet out of frame.
Edit: Changed my vote to a negative because knees out of frame shouldn't be nuked.
I created knees_out_of_frame almost 4 years ago to capture things like post #5556427. I populated it out a bit and made a wiki to describe what it was for. Basically, as HyphenSam says, an equivalent of feet out of frame but when it is the knees rather than the feet that aren't in the picture, and a key point being that the lower leg then came back in the picture. This is common enough that I felt and feel it warrants a tag, and isn't covered by any other tag.
I don't think it's a pedantic tag either.
I see some idiots have recently decided to ignore this and flood it with cowboy shot images, but most of the tag as a whole, rather than just looking at the most recent ones, is still what it was supposed to be.
Okay, with the knees out of frame issue out of the way, I'll give this one a vote up. I can't believe some uploaders are really using those tags to portraits, nuking all these tags doesn't sound like a bad idea now.