With the way they are hawking those credit cards on US flights, I'm surprised there are any zones after Zone 2.
Now that they are about 75% consistent on calling the F cabin to board prior to Zone 1, I think that they should group the Silvers with Zone 1, and classify Zone 2 for just the credit card holders. I think that Silvers, who still fly alot of BIS miles (with the exception of those who bought their status), should be able to board before the credit card holders.