The "change several bugs at once" page (a link to which appears at the bottom of every bug search result list) has no apparent way to add words to the "Status WhiteBoard" field in many bugs at once. I wish it did. Many groups use the Status WhiteBoard as an alternative keyword list. It is possible to add keywords to many bugs at once, but not to the status whiteboards of many bugs at once.
Since 2.22, the status whiteboard is present when changing several bugs at once. But it only lets you overwrite the existing content of the status whiteboard, not adding new stuff without removing the existing content. So this bug is not a dupe. :)
With version 2.20, I've noticed my setup is also "clearing" all the status whiteboard, not just overwriting it.