Cavalry has 3 purposes:
A. Anti-Cavalry
B. Suicide charging lines
C. Dismounting and acting as extremely mobile skirmishers
Best use in my opinion has always been C, due to Curassiers and Hussars often being witted down to uselessness after a charge or two, and Uhlans being poor in anything that's not anti-Cavalry.
Ahem, no.
A) In NW, cavalry shouldn't focus
too much on ennemy cavalry (that's a mistake we oftenly do in the 7e).
In a Linebattle, cavalry has to stay close to the lines and skirms in order to protect them. One of the easiest part of cavalry is when we charge a line that is engaged in melee with one of our lines. We simply have to 'bump' the ennemies to give them to our infantrymen, or to slay the ennemies in their back.
B) No. Suicide charges on lines never works. Even if you have skilled horsemen, and you kill more infantrymen than you loose men, your side will loose all of its cavalry, and they will get beaten.
C) There must be like 2 or 3 regiments of Dragoons, and they are far from being the best cavalry regiments out there. Dragoons can be useful when they dismount, but when it comes to the sword, they are getting beaten by nearly every unit, since their stats are pretty bad.
And you forgot some of the most important jobs of cavalry, destroying the skirmishers and the artillery. An army that protects these units badly, and loose it to the ennemy cavalry will most likely be destroyed by the other side's shooting.
In my opinion a good cavalry regiment must be good against pretty much anything. I always see regiments that are great at beating ennemy cavalry, but that don't know what to do when it comes to charging a line or something.
Like pretty much every unit, cavalry is a deadly weapon, but only when it's played the right way.