No, not everyone, but 95% will!
It would be good if you could only race against the same cars as you are driving, or if you choose to race a touring car for example, it has to be standard, no power upgrades and you race against the other 10 or so touring cars.
Yes, there will always be someone who will try and abuse or exploit the situation to gain an advantage, once they have done it, others will follow and so on.
If the rewards (money and points) are made too easy and more profitable than set races in A spec, people will concentrate on this and not the set races in A spec. If the rewards are too low people will not use this feature and the development time to implement this could have been spent on other areas of the game.
If you are making a race and deciding upon the prizes, it will be difficult to implement a fair reward system based on the huge number of different varables that can apply to a race, also you would need to make sure the rewards are in line with similiar A spec races, and that wouldn't be easy.
I think being able to create your own race then sharing it online for others to play would be a good idea and keep people coming back for a bit longer.
Currently online you can choose the track, car, laps, grip level, tyres, power, weight and use of driving aids (and in some instances, the weather too.
Yes, I do agree however, there are certian factors you can't choose, like FF cars only or Japanese cars pre 1990 etc (and these would be a good, and welcome addition to the restrictions you can choose from) but there is alot of scope already.
Generally, it isn't a totally bad idea, but when you look at the time it has taken to make and release GT5 and then look at the problems and issues people have with it, I just think it wouldn't work. If polyphony release a game with as many issues as GT5 then trying to implement something like this would just go horribly wrong.