For hard shell tacos, lettuce is purely for structural integrity. Place it on the bottom, and the juices from the meat do not infiltrate the shell and make it more susceptible to breakage.
AS far as Taco Bell goes, they basically ruined me for regular hard shell tacos. Thanks to their Double Decker(?) I have to have my hard shell taco enclosed in a soft tortilla. The flour and corn combo is bliss, and if the shell breaks, you have no loss of ingredients.
Also, not sure if they have it anymore, but the burritos that had Fritos in it was divine.
Note, as I am in WA state, despite our high Mexican population, we are far away from Mexico, so are content with our Mexican food tasting nothing like actual Mexican food.