[Esel Mann]

To be considered full-time for benefits, one must work on average 32 hours or more per week (how many weeks are used for the average escapes me for the moment).

What many companies are already doing, for example retail type companies, is working the bulk of their workforce under 32 hours so that they are considered part-time and thus it is not necessary to provide the benefits offered to full-time.
Not needing to pay out on the benefits side more than outweighs the additional part-time labor cost to cover their day-to-day activities.

So in essence what you suggest, is already being applied to some extent and has been for some time now. Naturally it is not being applied across the board because it is not practical in many cases for the reasons Red Beard pointed out.

Also to be competitive here in the U.S., full-time employment type jobs require that productivity offset the labor costs. To a company it's all about $$$. If they they could get sufficient productivity out of 30 hours for each full-time employee they would as the plant operation savings would be significant. However, that appears to not be the case, hence 40 hours remains the typical workweek. However, because the 40 hour work week has been around for so long, it's become engrained into our work habit. It's not a bad idea every now and then to re-visit it to see if perhaps one could reduce the work week and yet keep overall productivity at the same level!