I've been editing with FCP since 2001 for broadcast (and other purposes) and this is the first time I've ever read that "it's standard to remove the video portion of the clip and replace it with a video slug." I simply place the stills or b-roll clips on the track above the video and am done with it. Or you can overwrite the video with the stills or b-roll on the same track. Either way works.
I generally adda slug between the still image and the video image below if the image does not fill the frame, but there are many ways to skin a cat in fcp.