What makes the rolling starts in GT5P even worse is that in some events the lead one or two cars start 10-20 car lengths ahead of the next car. You can see this especially in the A class events with the GT-R and the F430 leading the pack. The R8 is so far behind them that they have a massive lead even before anything happens. Then you have to wait for every car ahead of you to pass the start/finish line, and only then can you take control (after the two lead cars that already had a massive lead get even further away). That's just a joke. It's not even racing once you start. You need to just plow your way through everyone. If you try to race cleanly and wait for opportunities, you will never win in just 3-5 laps.
Although PD may be worried about how to make a proper rolling start, because passing is not allowed in rolling starts before you reach the finish line. If they gave you control then you could just pass a few cars before you get there. At first I thought it may be difficult to use a penalty system or something with this (have to let driver passed, DSQ'd, have to restart, time penalty at finish, etc.), but then I remembered the live time penalty from Prologue. That would be a good way to do it. If you pass another car, your throttle is cut for 5 seconds.
Then again, I just thought now while writing this of something probably better than all of this. Just have the AI control your car and have a proper rolling start:
Start with a standard GT5P rolling start where cars are at cruising speed. Then as the leader goes around the last corner he goes on full throttle and every car behind him does the same, however all the cars have discipline and do not try to pass (even though it would be hard enough anyway in that short period of time). This includes the AI controlling your car at full throttle. Once you pass the start/finish line, you take control.
Controlling the car yourself would be a nice feature, but given what we've seen in that past in GT, that's extremely unlikely to happen and could also have other problems. Therefore the method I just mentioned would be the next best thing (it's also more of an equalizer).