I have CarHartt type pants, and I love them for working in the winter. I also love the bib top, adds warmth. For riding, though, there are several advantages to motorcycle overpants. First is the adjustable ankle, to keep the wind out. Work pants generally have a large leg opening, and no adjusting flap or strap. Motorcycle pants are more waterproof, too, with taped seams and folding rain flaps at the zippers. Most have an elastic waist, which helps reduce bulk when you sit on the bike. Motorcycle pants are made from Cordura, or a similar ballistic fabric, that won't tear or melt readily in a slide, and is abrasion resistant. An easily accessible thigh pocket is another advantage, as are the velcoed pocket flaps to keep your stuff where it belongs. Finally, some have removeable liners, so you can wear just the overpant in milder weather. Bottom line is that there are a lot of choices, and a person has to choose the features and price they want. What works for one, may not suit another.
-Scotty