Huh, learn something new every day. I never knew that they even made western half chaps. I always kind of thought that the purpose of english half chaps was to keep you from getting your leg pinched by the thin stirrup leather when you weren't wearing tall boots. That's not an issue in western saddles so you really wouldn't even need them except just for the look.
Though my country-fied mind keeps insisting that "western half chap" = "chink"