I will make it simple for you then. PD felt that a realistic simulations may be to difficult to use for casual players who may buy the game. Some people find games too hard and never play them agains, so PD decided have a realistic mode for the GT fans, and a arcade mode for the casual gamers, bearing in mind casual gamers make up the majority of sales.
I have absolutley no problem with PD offering two modes, i just wish the Sim mode was at least as good as other racers' physics that aren't even that new, like i've said before the tyre simulation just doesn't feel quite right yet. even with the noticable improvements.
You could argue however, that they could just offer one very realistic mode, and for newbies they could just add a lot of aids. (and lets not forget the licences, which can help people learn.)
Such as visual cues of where to brake and just have the car partially controlled by AI.
Another thing i wish, was for a more customizable AI difficulty.
I can see why a newbie would be happy to start in last place and then come out on top by the end of the race... but this is hardly any fun or realistic for an experienced user.
Basicly, it'd be nice if the AI controlled the car to it's limits. In all other GT's (and the demo too.) it's just slower on straights for example, even if the car it drives is faster yours, you can still pass them with ease sometimes.
It would be really cool if they added a qualyfing lap instead of just always starting out somewhere back, and pump up the AI to realistic levels.
Lets get real here, if you start at 8th place on an F430 and there's an F430 starting at first place you shouldn't be able to catch it in 3 laps unless the driver made a huge mistake.
I mean, sure it's progress that the AI will now sometimes give you room to pass or try to pass you with out bumping you, but this was just one the issues with the AI.
For this realistic AI they would need to come out with a viable way to progress in the game even if you're not finishing in first place all the time.
Hell, while they are at it they could come out with "superdrivers" that actually have a personality.
All this of course would be needless if PD delivers an online way to get trough the game from start to finish. (which in my book is the best idea, they could have a ladder system and in turn they could spend less time developing this system than trying to come up with convincing AI.)
And before it's suggested, i know that switching tyres results in a more fair battle between the user and the AI, but it's just because they can corner a lot faster than you can (they don't switch tyres at all.), and the AI will bump you as a result of this a good deal of the time.