There's something unique about all the cavalry classes? Lancers and hussars can both do the same thing, and the same applies to cuirassiers and unarmored heavy cavalry
The only real unique class is the dragoon, and it's unique simply by the fact that it's complete shite. It is literally a compromise mounted on a compromise.
Lancers: have lances. (obviousity is obvious);
Hussars: extra quick, a tad less melee damage;
Heavy cav: slow, but strong, sometimes with extra armor;
Dragoons: Has a bit of it all, which makes it a jack of all trades. I've played a few linebattles as French Dragoons, and when played right, they can kick ass in ranged and melee.
Which kinda makes every class different. Giving them all firearms, next to the few pistols they have now, will give very unbalanced classes. A cavalry player with a lance and a musket? What?
Every class already has the opportunity to drive-by. What every class (Especially the heavies) doesn't have is the ability to defend themselves from a drive-by. (Play as heavy cavalry for a few maps, and you'll see what I mean.)
How does giving every class more opportunity to drive-by protect them from drive-by's? By having all cavalry players charging at eachother while pointing muskets in eachother's faces?