Preheat a grill to medium-high. Stack the pitas and wrap in heavy-duty foil; place on the grill and cover to warm.
In a large bowl, combine the beef with the cumin, 3/4 teaspoon salt and 1/4 teaspoon pepper; shape into twelve 1/3-inch-thick patties. In another large bowl, combine the romaine, tomatoes and pickles; toss with the olive oil and lemon juice and season with salt and pepper.
Remove the pitas from the grill. Transfer the beef patties to the grill, cover and grill for 2 minutes on each side for medium. Spread each pita with some of the hummus, then top with some of the salad and 3 patties. Serve with the lemon wedges.