That's true. But if your car has trouble starting, or if you need to "clear out the plugs" by revving the engine, your plugs are too cold. If the tuner cannot make a tune which encompasses proper fuel mixture and ignition throughout the rev range and in all conditions, it's not a proper tune. You cannot really call a tune right if you have to switch spark plugs for normal driving and more spirited driving. If plugs are fouling, the plug is too cold or the mixture too rich. If you're getting predetonation, the mixture is too lean, the ignition too advanced, or the plug is too hot. That's really all there is to it.
But the way people talk about plug heat ranges it's as if they believe that the colder plug is always better and it makes more power. Yet the things they are having trouble with indicate that the plugs are too cold, and they'd be better off with the original heat range plug.