* Payroll taxes: OF COURSE they're saving the same amount on taxes! It's not like they pre-pay taxes on an assumed amount. It's a % of what they pay you. If they pay you less, they pay less taxes on your behalf.
* Workers Comp: Firms play insanely low rates on workers comp for programmers, and workers comp is generally calculated at (RATE for job class) * ($100 payroll allocated to job class). If you pay the employee less, your workers comp premium decreases at the same rate.
* PTO: If the company wishes to offer full time PTO benefits to a part timer, they may, but many businesses have separate PTO policies for full and part time employees, including slower accrual for part timers.
Training time and management overhead are probably the real costs here.
> In fact, more people working fewer hours increases most of these costs.
More people working fewer hours could also increase productivity (that's the point of this discussion) thus could increase revenue and profit, as well.
If you pay 10% more and make 20% more revenue, that's a GOOD DEAL