For Marans... I would say feather legged are preferred.
However, some folks don't want feather legged birds period as their feet tend to grab mud or poo. Especially easy to notice on lighter colored birds say Cochin or Silkies.
If someone is selling clean legged black copper... I would say value is lower... in fact I would (possibly ignorantly) assume they had some mixing going on and avoid them all together... noting how much black sex link hens can look like clean legged BCM.
I rarely ever see feather legged cuckoo.
I would say most of all it's important to know YOUR local market. Folks in my market can get clean legged birds of soooo many varieties. And French standard... is what all of MY patrons were seeking. I would not waste my time with clean legged and consider those to be more hatchery quality in general (doesn't mean it's true, but sharing MY perspective).
If you are breeding or selling ANYTHING... what I discovered it's key to work with what YOU like. Otherwise it becomes a less desirable adventure. After all you have to look at these birds day in and day out.
If you do go clean legged with Marans... I would say make the point about breeding to the English standard, as some folks don't realize their is more than one standard. For me, Marans aren't renowned for their English traits and it's their French traits that set them apart... ALWAYS said with S at the end (whether single or plural) like the region they hail from in the port town of Marans, France.
While (seeing your in TX) feathered legs might seem hot in warm weather... the feathers actually shade the legs, I think.