Mmmmh ... usage depends on faction. Playing as GB, my 2 Yeoman Squadrons from start get very old. I use them only for recon and chasing routers.

As France or Austria i always had 1 or 2 lancers ("ULANEN") in a stack, just for the "feel" and to engage other cavalry or artllery. Playing as Prussia,
2-3 units of hussars are always in my army. For the same reasons as the lancers in austrian armies.

All my armies are mostly made of line infantry and 12-pounders and howitzers, maybe 2 grenadier units - that's all.

And my "Jäger-Feldwebel" at the army always said: Infantry is the queen of the battlefield!