Agreed, but maybe the other side (or any of the hops in between) have some weird and/or incorrect implementation on which the CRC/Checksum fails because the bit is set? I'm not sure what would happen if the bit is excluded from the CRC calculation but included in the check against/with the CRC for example. Would be a nice edge-case to test and, ofcourse, shouldn't fail. But who knows, maybe it does somewhere along the line?

Also, as barosl observes (https://news.ycombinator.com/item?id=10633361): "but I guess if one is following the RFC rigorously, dropping packets with the reserved bit enabled is the correct"