Interestingly, I think the depth of tuning available is what makes Forza a cut above many of its competition. If tweaking this or that had no effect on the car, then it would all be rather pointless.
I don't criticize anyone for buying a SF tune. As you say, when it doesn't quite work right it's a handicap. When you build your own tune from the ground up, and it's successful, you get a feeling of accomplishment and more personal ownership of the car. It gets its own personality that you helped to create.
Yes, I've been destroyed on the track many times by drivers who are superior tuners. It's a skill, like anything else, and if I can't both tune AND drive better than my rival, then I didn't deserve to win the race.