If Ford wants to put a 1.0 liter Turbo 3-Cyl in a car, they are more than welcome to.
It just better get phenomenally better gas mileage and not be a high cost option. If it's going to have roughly the same output as the 1.6 NA 4-Cyl it has to have some sort of appeal.
Actually, the more I think about it, I can kind of see them using the 1.0 EcoBoost in a "SFE Model". Do something similar to the Cobalt XFE (except it's not a sucky Cobalt) and have a special model for getting better mileage. Maybe keep some of the heavy options to a minimum. No 17" wheels, less sound proofing, no power windows, no sunroof, use the SFE package aero tweaks, require the PowerShift, etc...
They should also use a Fusion Hybrid style IP to show how efficiently you're driving.
However, I'd rather they build an ST/SVT/SHO version with the 1.4l EcoBoost that produces around 170-180HP. I'm sure there would be more profit (though maybe slightly less volume) in it. Especially since it would most likely be an optioned up Fiesta to begin with. If they can do that for under $25k, I'll pull out my checkbook.