Sure you can cheat with the licences, and buy the best car for each championship and whack a stage 4 turbo on it and blow away the opposition, and if thats what floats your boat then fine, I don't have a problem with that. In the early days with GT1 thats exactly what I would do, and I had a blast. Seriously, it was great fun.
But by the time GT2 came along I was getting bored. Blow away the opposition in a super souped up car and win every race, wheres the skill in that? I started to try and win races with a car no better than the opposition and try and out-drive them instead. At least it was more challenging, and I felt better about it, even a sense of pride.
Bear in mind the licences are by far and away the hardest part of this game, I practised for weeks for the GTWC on the Toyko circuit before I could manage a win there, but the same lines I used are only good enough for a Silver at best on the S-licence test and usually a Bronze. Also remember that the licences are usually accumulative, the skill to take a particular set of corners in the right way will occur again and again through the tests but it will get progressively more difficult with faster or poorer handling cars, and usually culminating in a full lap of the circuit. If you don't get it right early on you'll find yourself increasingly off the pace. So if you're having a problem with a particular level of test, go back to the earlier, slower version of it and try and get a good time and you'll find it a lot easier when you go back the one you were having trouble with. Also practise. A lot. In GT1 I remember a International licence test involving a TVR Griffith on the Grand Valley Speedway that I simply could not pass, I tried over and over again for weeks on end until I eventually went and bought a Griffith and took it on a Time Trial on the GVS and just went round and round, over and over again for what seemed like hours. Once I went back to the test I passed it on the first or second attempt...
Apart from the licence guide on this forum one I thing I find incredibly helpful is there are videos on YouTube of people getting Gold on the licence tests and unlike the demo's you can pause and rewind the video, and if you're having trouble with a particular corner on a full guide lap you can watch that point over and over. Also these guys often take different lines and get better times than the demo run so I'lll try their lines instead and once got three Gold's where I simply couldn't manage it before.
But at the end of the day it's your game, you paid good money for it, and you do whatever the hell you want with it, and don't let anyone tell you otherwise.