Audi currently uses a tipronic box, but if porsche was using this style of transmission, they would be using a pdk (porsche dopplekuplung (spelling?)), which works a little differently. Not sure if tiptronic is a dual clutch transmission, but the pdk definitely is. A dual-clutch tranny uses two shafts with odd gears on one, and evens on the other. Each shaft has a clutch, and they can have one shaft preloaded with the next gear, and just switch between clutches to get ultra fast gear changes. These transmissions aren't very well suited for race applications though. They are more complicated, and heavier than what's currently being used in motorsports.

A sequential gearbox on the other hand is very simple, can take tons of punishment (compared to a tiptronic box, or a dual-clutch box, which are essentially the same as the automatic transmission you'd find on any other car, but a little tuned up,) is lighter, is simpler to use (you get a nice thunk with the change, so you know you've changed, even in the heat of the moment,) et cetera. It's pretty much a manual transmission with the synchros taken out so that you can change gears without depressing the clutch. While you don't need to use the clutch to change gears, you still need it for a launch. Even formula 1 cars use a clutch, it's just that they use a pair of paddles (one is full depression, and the other is at the bite point for the start of the race launch) as momentary switches to depress it.