The diluted ammonia solution that is used in household concentrations is actually a weak base, not a strong base. And just because it is basic doesn't mean it works the same way as phosphates etc. Heck, if all you wanted was a strong base you could just add sodium hydroxide (lye), and then deal with the tattered fabrics it left behind.
Sodium carbonate (washing soda) is a stronger base than STPP. Stronger still is sodium silicate (water glass). Both sodium carbonate and sodium silicate are common and cheap ingredients of powdered laundry detergents. Their limitations include harsh action on fabrics, and their tendency to form insoluble precipitates with common water and soiled laundry minerals, resulting in limestone being deposited on fabrics and washer parts.
STPP, on the other hand, excels at lifting dirt and stains from fabrics more gently than the others, as well as keeping it soluble so that it can be rinsed away without redeposition on laundry or formation of limestone deposits inside the washer.
As far as I can tell, ammonia is a good grease cutter, and it has disinfectant properties as well. I'm not sure I'd want my washer smelling like it though, esp. since the laundry closet there is right in the middle of the home, in an area open to the kitchen and family room. Also, ammonia may react with certain materials and cause them to darken - a reason why it is used to darken wood or leather that contains tannic acid.
If you really really like ammonia... eat fast food burgers and school lunches. Apparently someone had the brilliant idea that beef scraps from the outer surfaces of a carcass could be reclaimed by exposing them to an ammonia vapor process, killing the large numbers of pathogenic bacteria that they might otherwise contain. Previously these scraps would have been made into dog food or soap. I have more than a little problem with this, as consuming even dead bacteria means one is ingesting large amounts of the bacterial cell walls, etc, that may contain allegens that can make you feel sick anyway.
Apparently the practice is so widespread that the only way one might avoid the stuff in a burger or meatball may be to buy beef from a butcher and grind it yourself.
Enjoy your Big Mac!