You can selectively have patents for the tier-2 designs and use trade secrets for the tier-1 designs that you wouldn't really want to diverge to the world. It's

You can selectively have patents for the tier-2 designs and use trade secrets for the tier-1 designs that you wouldn't really want to diverge to the world. It's not mutually exclusive like you're implying here (okay, in the strictest sense a certain part of the invention can only be either be patented or kept secret, but the totality of the device makes it likely that both patented and non-patented trade secrets were used).