the simple answer is that if you keep the car long enough you will pay more for repairs. downshifting through the gears every time you slow down is eventually going to cost more than using your brakes. which is cheaper to replace... the brakes or the transmission? your car will need to downshift on its own anyway but it is the fact that you will be downshifting into much higher rpms than the transmission would do automatically.

shifting through the gears with the autostick might provide entertainment to the driver but your car will not accelerate as quickly because of the time delay in the shifts. so from a performance standpoint you probably aren't gaining anything.

the autostick can be useful by allowing you to select a more desirable gear when planning ahead to accelerate out of a turn, up a hill, or from a crawl, since your transmission doesn't know what's expected.

but as far as trying to use the autostick full-time as a manual is concerned, it isn't going to enhance performance and might wear out the transmission faster.