Elite martial artists generally have no interest in serving as soldiers. After all, their superior skills offer them many much more attractive options. I do wonder, however, if most of the national armies we see in wuxia have small squads of "super soldiers" consisting of low-level wulin fighters ranging from the levels of Yeung Teet Sum and Gwok Siu Teen to the weaker Gong Nam Freaks for special missions that don't warrant sending in a Greats-level fighter (which the national army probably doesn't have at their service anyway), and which standard soldiers lack the ability to accomplish, but are well-suited to small groups (e.g. a dozen or so) of low-level wulin warriors. Essentially, these fighters would be the ancient equivalent of Navy SEALS, Army Rangers, or Green Berets.