I agree, any pricing plan depends on character to be applied fairly.
Now I am about to say something that is purely personal opinion formed from what happens in this area of the country that I work.
I readily admit other areas may be (likely are) different.
It has been my experience that the workers that are paid by the piece in this area
are generally not of the same character of the trades that pay by the hour.
The workers that these 'by the piece' trades use generally have drug or alcohol problems, are ex or future cons. and are transient workers.
They only care about getting done and getting their money, they do care about quality as when that job is done they move on to another employer.
Of course there are always exceptions, however the above is IMO true of the majority.
Sorry if I offended anyone, if your area has a different situation then good for you, I wish it was that way here as well.
Here in my area when I see these workers arrive on the job I take more care to lock down our tools.
Again understand I realize that other areas may well have different conditions.