Ya the way the passes work, you can't just drop a segment, that ticket has to be cancelled, released into the air and attempt to pull it back down.

IE: Let's say SFO-YVR was completely sold out, or worse oversold, you'd be crazy to cancel thinking you could book it again, cuz even if you booked again immediately you might not be able to if the airline is already oversold to a number that they'd like to shrink.