True as per their current defence capabilities. However the new Arrow defence system which has almost been developed has multi anti-missile defence capabilities.
But there is always a margin of error, and you're right, a missile could get through. Nevertheless, I fully doubt Iran would ever actually fire a missile at Israel unless it was in response to one being fired at it. They wouldn't even do it in response to a full invasion, imo, as they'd know the implications of it (also, the nuclear fallout would probably drift over Iran as well). The only real danger of Iran having the missiles is if they were to get into the hands of terrorists.