Sole reason you should purchase it (though there is a package that comes with the SA and an AAMC test.. so there's no need to purchase it individually).
The questions are THE MOST reflective of the current MCAT, they are specifically designed for this purpose. Moreso than the AAMC FLs.
I wouldn't buy it over the SAs, but I would probably buy it over the earlier AAMCs.
IMO,
AAMC 7-11
AAMC SAs
AAMC Study Guide
AAMC 3-6
if I had limited funds