The person at the front at are airport is three times more important then the said wander.

If the person up front can make sure you have nothing else on you, and load the belt so reruns might not be a problem, then it keeps a lane running.

Now if you get no one out front and you have a mix of FF and new PAX, its a problem. Because a lot of PAX don't read those signs and never have. They come up and stand there waiting for instructions and hold up the line because of it.

A person up front can play both the role of entry and wand, while helping to speed up the process.

And personally, if it was three screeners at a lane, I would want a Xray, Walkthru and a bag check. Bag checks are more important in my view then any wanding.