Yes, because the oil pump is bolted to the lower case half, and cannot be safely removed without taking off the clutch basket first. Yes, you could unbolt the pump and wiggle the pump plunger out, but it's not good practice.Do I have to remove the outer clutch basket and oil pump before I can open the crankcase?
The oil spinner has to come off in order to remove the crank gears which mesh with the clutch basket gear, you need a special tool to undo the crank shaft nut inside the oil spinner. Use a handle bar bolt ( or similar ) to draw off the oil spinner cover, don't try pulling it out with pliers.
On the other side of the engine, the generator rotor / starter clutch does not need to be removed.
When separating the cases, remember that there is hidden bolt beneath the oil drain plug. This caught me out first time I had a 175 apart.