Too much water softening agent/alkali is just as bad as too 
Well, that might be true for precipitating washing soda, but even then the method of action (or reaction) is entirely different than having too hard water.
The harshest, strongest alkali in common use in home laundry is water glass, or sodium silicate. It's added to boost alkalinity and also to help "protect washer parts", presumably by inhibiting deposition of lime scale inside the washer.
Next up is washing soda or sodium carbonate. It's the most common ingredient of modern powders, especially since the relatively innocuous filler known as sodium sulfate has been reduced or removed from concentrated powders.
Complex phosphate or Sodium Tri Poly Phosphate is the most gentle of this trio, at least in terms of alkalinity. I've gotten my hands in a solution thereof, even taken a bath in the stuff, without any skin irritation (and I felt super clean thereafter ;-)...
It should also be noticed that STPP is also a quite common food additive, as it helps various foods retain moisture and texture (like dried fruits in cereal or packaged luncheon meats). 
One article I read posited that STPP in the waste stream is actually environmentally beneficial since it can act as a buffer and counter the toxic effects of acid rain on ponds and streams.
Of course, there can always be too much of a good thing. However I'm not in total agreement that slippery water by itself means enough detergent has been added. The slipperyness simply indicates the level of alkalinity. It doesn't distinguish between precipitating and non-precipitating alkalinizers. It doesn't give any indication of the adequate level of other components, such as enzyme, free dye inhibitors, etc.
In my experience, a 30% mixture of STPP to Sears Ultra HE Plus works great if enough is added to result in a very small amount of suds. For some loads, that could be as little as 1 oz. For others, it could as much as 6 oz. It is a learning process, guided by results, not so much by theory or rules.  
Detergent mfgs used to add 50% STPP to their laundry powders. Modern advice is that 30% is sufficient, and helps reduce the amount released into the environment.