Originally Posted by Beerman
The benefit of a driver is for freestyle riders: they require almost no maintenence; you put it on there, and you never have to tighten it or anything.
The benefit of having cogs is for racers, so that they can very quickly swap their gears; basically, it's the same idea as having a spider up front.
Cassettes with separate cogs and lockrings still have drivers (just like freecoasters), that's the part that the cog slides onto. one-piece machined cog/drivers came into being because teeny tiny nibbler gears up front mean even teenier rear cogs and the drivers for existing cassettes wouldn't take 9t cogs...the driver is physically larger than the cog would have to be. One-piece units are a workaround so companies didn't have to invent a smaller driver standard or heaven forbid a new cassette. The other issue was durability...racers usually don't run compact sprockets, which means the cogs are larger, thicker and wear longer. The fewer teeth you got wrapped around a standard sized driver body, the thinner that cog gets and the more likely you are to wear it out or snap it.