I prefer to use the Chassis Reinforcement on virtually every car, as I find that it makes the suspension behave more crisply, especially with turn in. Many will argue that it "tightens" the car up, but I find you can tune around that. Without the reinforcement I find steering response feels "sloppy"
I will add to this point.
Most of the population does not like to use this, but the main reason is that initial testing on it was done early in gt6's life. So many things have changed since then/major patches/updates.
I agree with this being useful now.
Here is what I will add though;
1st of all, never ever ever do a reinforce in the middle of tuning.
2nd, and this is only possible if the car is cheap enough, I find the best thing to do is tune the car fully without reinforcement. If you are completely happy at this point, then no point looking farther. But if not, and its not expensive, buy another car. do the reinforcement, then tune it like a brand new car. don't even look at your other settings, just tune it like new.
Then compare, lap times, feeling of drive, tire wear speed, ect.
Many times I have found I liked the reinforcement version a ton better.
But because of the cost issue, I shy away from reinforcement of high priced cars.