That's an old rule in poker. Tipping your hand lets the opponent see what you have and what it is worth. If you only have ONE iron dome unit and it is currently deployed at Beersheva, then the enemy automatically knows not to waste its rockets in that direction. The real solution is to have several units that can protect all the areas that can be reached by rockets from Gaza. Since there is unquestionably a significant overlap of the units coverage, relatively few units are needed. Even if the units provided are insufficient to cover the northern parts of Israel, the ones deployed in the south will stop the threats from Gaza on the areas south of Jeusalem. That will free-up some of the IDF assets to use elsewhere.
US rally shows support for ex-Marine held by Iran on spying charges (AP)
from the article: What about the Iron Dome?