You get top-tier only by flying the requisite amount (on QF and BA it is fare based rather than 100K miles, which is good if you fly in paid F but not so good for discount fares). The advantage of non-AA Emerald status is access to the AA lounges (AC and FL). The other reason to have top-tier status is priority in reservations, guaranteed seats on sold-out flights and help during irregular operations. Some of us travel on a strict schedule and really do have to be on a specific flight, making high status on multiple airlines essential. Obviously this is at the cost of sub-optimal mileage allocation, but sometimes status is much more important than mileage.