The value of 6 credits can be very high when you live in a city (usually in the U.S) where AC has a large price difference between Lat-B and Z fare for TATL, and you're E or SE.

If your travel date is somewhat flexible, Lat-B with 6 credits gives you confirmed upgrades at purchase with available R space (when you do it far in advance there's usually some R space). Since Lat-B is fully refundable, it has more flexibility than a Z fare - and you *can* buy it far in advance with the peace of mind that you can always get a refund.