Aside from the bizarre spelling of their name Musketiers still retain the same roles as Musketeers (and musket line-type units) in other factions but they differ in two places: the first is their cost; the second, their tactical parameters.
As noted, Musketiers for Austria and Bavaria are cheaper but somewhat weaker than the normal vanilla Musketeer, but that doesn't diminish their value in the eyes of any commander. Any player of Rise of Nations worth their salt will take advantage of the cheaper cost to pump out as many of these units in order to meet their enemies on the battlefield. This is especially more so since Musketiers tend to have an additional +3 hitpoints, meaning that in some borderline cases, they may survive far longer than they normally wound in a straight-up fight. As for Prussia, the Musketier retains the same cost structure as the Musketeer, but is a slightly superior unit, owing to its better hitpoints and superior firepower — in fact, this firepower could well make it the best unit of its kind but for the existence of the Red Coats.