My understanding is the chassi flex is it helps in two ways. Firstly softer chassi just like softer suspension increases grip, and a combination of a softer chassi allows for slightly stiffer suspension which provides a more responsive car with the same level of grip.

The second advantage is a softer chassi helps deal with the high frequency vibrations which the suspension cannot absorb and it transmitted into the chassi.

If you go in a modern day very stiff car with poor shocks there are a lot of high frequency vibrations which are transmitted throughout the chassi and can be felt by the passengers. This can result in less stability under load in corners. With great shocks this is much less of a problem but we don't have that in RC as of yet. This could be the next RC evolution.

Also keep in mind most bumps on RC tracks are much bigger than they are in 1/1.