I agree with you that all that matters is what comes out of the speakers, but no matter how great any programmer is, if the engine doesn't have the power, there will be limitations in what you can do, like playing back 2 amps at the same time, while using high quality effects etc, all at the same time, you need processing power, I admit most of my patches are very simple, but occasionally i might need a few patches that really push it to the limit, and it's good to know it's there, for some players who like to use lot's of effects it can make a difference.
Edit: