Do not confuse balancing V-8's as compared to 4 cylinder engines. It is a completely different process. Since two cylinders are directly opposed to the other two cylinders, putting bob weights on the crank does not make any difference in the balancing measurements. The added weights just cancel each other out when spinning the crank. Aftermarket pistons and rods come balanced to within a gram. (That is very close when you consider there is 453.6 grams in a pound.) Clutches and flywheels also come spun balanced from the manufacturer. This is necessary in order to have their SFI certification. Same with aftermarket balancers.

The crank is balanced by itself with no weights attached to it. It does not make sense to add a clutch and flywheel to the crank when balancing because then you are stuck with the same flywheel and clutch forever unless you want to throw your engine out of balance or rebalance your crank after each clutch change.

As to adding the balancer to the crank when balancing, this is not done because you would rather read the out of balance condition on the crank itself and correct it rather than having the balancer mask that condition.