I don't really agree with this rule. My thought is that, you send a meat shield of your army respected low cost troops (conscripts, cultists) and your expensive troops behind them. As long as the shooting player can see 50% of a unit without any other unit in the way, they can shoot at them. But otherwise, they are more concerned with the hoard of troops coming at them. The only units that are exempt from this rule are and snipers (scopes) blast template weapons, etc. Just my idea, but Ian was right btw, they get a 5+ save