Just asking.....when an engine is dyno tested, why not run it as it will be run in the car, ie, alternator, water pump, power steering pump (if using), ac (if using)? That should give a real number, right?
If you're looking for data on the extra load on your engine, you might as well include your drivetrain as well. There's no reason to care about your engine accessories if you're not going to take your trans/driveshaft/diff/wheels/axles into consideration too, and they'll likely draw much more power than your water pump and alternator. At that point though, you'd want a chassis dyno with the engine in the car, not an engine dyno.
Maybe later you'll remove the power steering, or you want to put AC on the car, or you slap a different tranny into it, but your flywheel horsepower is always going to be your engine's "theoretical max HP" before everything else is bolted on to reduce that number. Sometimes it's just nice to know the HP you started with!