Well, for one thing, the car WILL NOT shift any faster or slower in manual mode than it would in full auto. Man-u-matic shifting was basically created for those that don't want the hassle of a clutch but would still like to feel like they can be more involved in their cars operation, if so desired on occasion. Honestly, most people I know that even have the option use it maybe a few times a year. Maybe.
I'm not 100% positive, but knowing how TQ and HP curves are designed by nature, I would assume that leaving it in a lower gear longer at higher RPMs would simply keep you up higher in the TQ/HP band, thus meaning you'd have more power on demand. This might be useful in situations such as preparing to pass someone on the highway, as a downshift before you pull out to pass means that you'd immediately be in gear and ready to deliver the power when it's immediately needed...much like how you'd downshift a manual. All that mode is simply doing is giving you more say so as to when the car shifts...not at all HOW it shifts.
You should be starting to see now as to why fuel economy tends to suffer in manual mode, as automatic mode will attempt to give you the best mix of the appropriate power needed AT THAT MOMENT while achieving the best fuel economy possible through lower RPMS. Sitting in manual means you are taking away the computers ability to maximize fuel economy in exchange for faster delivery of power as you tend to sit and shift at higher RPMs then necessary in normal operation. Other times it might be used is in adverse driving conditions, such as snow. Many times, a downshift can be a safer way to slow your vehicle down vs. hitting the brakes and possibly locking up thus losing control.
Lastly, I don't think you're honestly going to be able to accelerate any faster in manual mode than in auto. You may be able to rev higher, thus creating the "excitement" illusion that you're moving faster, but in all reality, the car is just making more noise and likely using more fuel than it needs to. Again, it's just a way of making you feel more "in control" of you car byt allowing for more interaction. Stomping the gas in full auto will likely give you a quicker 1/4 mile time, as the CPU will be able to better gauge the best time to shift to the next gear, thus putting you in the power band again for the next gear down.
Sorry for the lengthy explaination...it's just not a short winded topic without getting confusing.
