Actually, only Russians didnt used cuirass for cuirassiers.. and later they put it back.. Cuirass was quite useful to have, as it could actually stop a bullet fired at medium or long range, while it protected from all sword slashes, which gave Cuirassiers and edge over other cavalry. But of course, Cuirassiers were heavy cavalry, were usually tall men on large horses.. therefore in melee had a huge advantage over others due to height and reach advantage.. but as Steph said, grenadiers didnt used grenades as part of their battlefield tactics anymore.. they evolved into specialized Shock troops.