SS,
I realize you are going by your personal experience. However I'm relaying information I've learned over the years, not only from my own personal experience but also from expert sources.
What the experts say: STPP performs three major functions in a laundry detergent:
1) Water softener
2) Break
3) Alkalinizer
STPP these functions without forming a precipitate when it encounters various minerals. It doesn't matter if those minerals are in the tap water or in the dirt on the laundry.
In detergents that replace STPP with sodium carbonate, it is necessary to add a anti-redeposition agent to avoid having the mineral-carbonate precipitate from being redepostied back on the laundry. So, you can add a fourth function to STPP:
4) Soil anti-redeposition agent.
Additionally, STPP is a much better break than sodium carbonate. A lot of this is due to what might be called the nano action of STPP, whereby on the molecular level its ability to keep dirt in solution means that it more readily coaxes dirt from fabrics than sodium carbonate. This is irregardless of the water hardness level.
If you don't general truly dirty laundry, and have soft water, then yes, you may never see the difference between detergents with vs. without STPP. However, even in soft water, (4 grains) I can see a huge difference between a detergents ability to remove all sorts of heavy soil and staingin from fabrics when STPP is the variable. I know I'm not alone in this observation; many others here have stated that they whites are much whiter with STPP added. In fact, that's how I first discovered the value of STPP. I noticed that my whites were no longer getting clean when my favorite detergent (Clout) dropped the use of STPP in the mid-90's.
Additionally, enzymes cannot replace all the functions of STPP, and they are basically useless against inorganic mineral-rich soiling and staining.
Q
Servant quarter style
Q