The old WCK maxim about bong sau never stays (bong but ting lau). Doesn't that really apply to every Wing Chun "hand"?
If tan sau, bong sau, fook sau, gum sau, lan sau, etc. are actions rather than positions, then should all hands be in a constant state of motion?
Otherwise you are just offering a lever for the opponent to control you?

I know, perhaps you can say that your gum/lan is controlling the opponent so it "stays out", but would be interested in your thoughts.