Hmm - I've always refereed to them as dust covers, and if you live on a dirt road and your house gets dusty, they really do help to keep that dust from getting up on the machine while it's put away.
All my treadles have something like that, and most of my other cabinets do too.
Like SteveH, I have always heard the Skirt Guard refer to the cast iron part that covers the edge of the flywheel on the treadle, so your skirt can't get caught between the belt and large wheel. Sort of like a chain guard on your bike that helps to keep your pant leg from getting caught in the chain - Some of us are old enough to remember dealing with bell-bottoms and bicycles!