Anyone needs a chassis plate:
On a more serious note, all the chassis plates bend. If you never experienced a bend chassis there are 3 options:
-You didn't check it right
-Setup board is also bend
-Car has been sitting on a shelf gathering dust
With the current cars it's also a lot easier to bend a chassis plate by all the flex which has been introduced. You can bend a chassis by hitting a curbstone too hard (big curbstone).
Having a flat chassis is one of the most important parts of car setup. You can set your downstops and all other settings perfectly but if everything is setup on a twisted plate all the settings will be off.
I check them by completely stripping the car and placing the upper part on a sheet of glass or carbon fibre.
A slight bow is still acceptable, but if it rocks from side to side by only the slightest amount I replace it.
If the chassis is twisted by only the smallest amount it will be more by a millimeter once you reach the wheel.
Another reason to replace the chassis is wear. Most tracks have one big sweeper which really grinds the chassis down on one side. Depending how low you run the car and with what size of tires you end the chassis can already be down to 2mm on one side after 3 or so races. This also messes up your downstop measurements.
As a disclaimer, I'm really anal about car setup, it has to be perfect.
If I could afford it would run a new chassis every race.