It's a matter of opinion. People post this idea every now and then. Many people here are already doing something similar to varying extents. Not everyone who finds the game dull or infuriating or even easy is using the fastest cars on the best tires for every event(not that I'm at all suggesting the OP is making that assumption).
The problem is that more difficult is not always the same as more fun. I learned this very clearly playing GT5 which I was able to make quite difficult but rarely fun(in A-Spec or Seasonals at least).
My own opinion on the matter:
Sharing the track with GT's basic AI and the lines they take and their intermittent ability to see you isn't terribly fun to begin with. Adding the ridiculous brake-checking we've had since mid-way through GT5's life makes it very un-fun. Add the ridiculous swerving they do in GT6 and it gets worse.
"Racing" against opponents whose pace is substantially dependent upon how far behind or ahead of the the human player they are is not fun. GT6 pushes this to an extreme.
Flying past ten cars that can't even begin to drive at the same pace as you, yet you are supposedly competing with, is fun for some people but very un-fun to others. Being required to do so in order to chase down one car that is significantly faster than them (but still significantly slower than you unless you only gain because of rubber-banding) is not fun to me for sure.
Not having any idea how close to the PP limit your opponents will actually be beforehand is not fun. GT6 only gives you 3 potential opponent cars without telling you their PP, and very often those cars are not at all indicative of what cars you will actually be racing against.
Getting rammed, shoved, cut off, brake-checked and spun is not fun.
Having to jump straight into a race with no opportunity to get accustomed to the combination of track/car/setup/opponent's pace before it "counts" is not much fun. Finding out on lap 1 of a 6 lap race in the fourth event of a tight championship that you have made a silly pre-race miscalculation - easily identified with just one practice/qualifying lap - that completely destroyed your chance of a decent finish is not fun.
And finally, having absolutely no idea whether gaining a second per lap will get you any closer to the leaders or not because you have absolutely no idea how close to their actual pace they are running due to the aforementioned rubber-banding is very much not fun and makes finding extra fun from extra difficulty incredibly frustrating.
Slowing the car down makes it more challenging for sure(as long as the rubber-banding doesn't thwart you). This can create an increased sense of achievement, and also means you'll make a bit more money as you go through the career from losing races. So it's not without merit. It won't make what you are actually doing during gameplay much more or less fun however.
Driving the cars in GT6 is lots of fun for me. Accomplishing a difficult task, particularly involving racing, is fun for me. Unfortunately, competing in the events that they put you in in GT6 alongside the AI drivers they give you is where the problem with lack of fun arises, and making that more difficult doesn't make it any more fun for me... at least not anymore.
To me, GT Career mode is simply work -- for completing once and then using to make money either by just re-running for cash or to make a little money while driving a car I'm interested in driving. If I want to have fun with the game, I'll race Arcade(which despite having some new problems on top of it's old problems can be fairly enjoyable), or I'll hotlap alone, or I'll go online(hopefully they'll sort the bugs out of that soon), or I'll run an endurance race assuming they ever put some in and don't completely screw them up.
I spent forever on GT5's career mode attempting to make it difficult and have fun with it. I eventually realized that I would have been much better off flying through it with easy cars and then farming seasonals for cash, thus saving vast amounts of time for those things that I could have fun with... or just for playing rFactor which is almost always fun.
