[A_D_E_P_T]

Ultimately, I think that it depends entirely on the type of product and how it's used.

If you take an aluminum can from 17g to 12g, that may represent some cost savings in manufacturing and transport, but the average soda drinker won't notice a difference.

But if you take an umbrella from 1700g to 1200g, that's the difference between something that's entirely unusable, to something that has practical utility -- if only barely. 500g would be much better. All else being equal the optimal weight for an umbrella is probably around 100g. Enough to know it's there, but not enough that extended use by the fifth-percentile human would be difficult or metabolically demanding.

If a human has to wear or carry it, and if there's a meaningful weight/comfort threshold associated with the product's use, every gram counts. Duct tape and aluminum cans don't fall into this category -- but, at the same time, this is why hiking and camping equipment tends to be extremely light, and why the athletic shoe companies keep researching lighter and better foams.

The problem is heightened in aerospace and automotive engineering, where fuel economy mandates tend to impose hard weight caps -- and getting a design in at well under the cap is a real engineering accomplishment. Offhand, I recall hearing that there was once a program where ~$15M dollars were spent on efforts to make a commercial airplane lighter. This resulted in about 20kg shaved off the aircraft's weight. That doesn't sound like much of a value, but the program was considered a great success.