Admittedly, I've not managed employees in a corporate environment, but I have in a call center (granted, a rather small one). In that situation it was drastically cheaper to hire two or three part timers than one full timer. Perhaps this is due to the state we were in, but the taxes we paid weren't large and part time employees didn't qualify for unemployment unless they worked more than 35 hours a week. What with income tax actually being taken from the employees wages and not an extra expense for us, it worked out like follows.
12 part employees making $9 an hour, could work a total of 30 hours each per week, equaling 360 work hours.
For every employee we had over 3 the state treasury charged us $35 per quarter ($85 for full time), minus the 12 part timers we had 1 owner and 2 managers (yes, small call center).
Since we paid no other fees at all, no medical and no taxes that weren't deduced from the employees wages, and the cost of 8 extra telephones and smart terminals was tiny ($250 or so for each, and in 4 years we had to replace one, once) the cost was roughly $170,160 a year. Hell, let's pretend we bought new phones and fancy computers every year for, say, $800 a piece. Add that in and it comes to $179,760.
Now, Had that been 6 full timers, granted we'd have bought fewer phones, but, each one of them would be getting 20 hours a week at time and a half, each working 60 hours a week.
Plus, they *do* qualify for unemployment so we'd have had to pay into that, as well provide them with some manner of health care most likely (who works 60 hours a week without benefits?).
Let's add up the basics before even bothering with unemployment, 6 people at 40 hours regular pay plus the 20 at time and a half comes to $196,560...
Wow, haven't even stated adding in state and federal fees and part timers are cheaper *after* fees for the same number of work hours by $26,000 ($16k if you figure the new machines) annually. The costs to the business for 12 even full time employees from the state/fed don't even remotely come close to costing more than $20k a year. Start adding in those fees and the costs just go up.
In most work environments, part time is cheaper by far. That's why part time exists. But, if you have a desk, and an access card, and your own email address... remember, you are not in an average work environment.