I noticed the recommendation to use curves in the manual. What's the reason behind that, isn't aircraft response realistic with straight curves?

I don't know why the MiG-21 specifically is like this, but I can think of reasons why it would be useful in other planes. Some aircraft (like the Citations I fly) have an aileron/rudder mixer that gives additive rudder for whatever aileron deflection you are putting in. I don't think it is linear, so it might make sense to make the curve progressively steeper (to a point) for larger control inputs. Again, I don't know why it is suggested for the MiG-21 though...

BeachAV8R

Generally speaking when you have the stick close to center you're going to want to make small adjustments. You don't want to sneeze and make yourself blackout or redout!

For comparison, your car has a curved response for its wheel. It is not linear.



The Jedi Master

Other aircraft may have been designed with a non-linear response between stick and controls even though the input appears linear in the axis options.

If the MiG-21 has a purely linear relationship between stick and controls, that might be why the curves have to be added in the options.