Sounds like expensive and inefficient way to test air defence system .... still they (Su-57s) hardly make any difference militarily, in fact I think basically the same as Feb 8, 2016
(and their mass production would prove me wrong)
The most likely reason of the existence of Su-57 should be the same like for the poseidon/ nuclear cruise missile and so on : increase the money that everyone has to spend for defence systems and sensors, and decrease the money available for attack systems.
It is the inverse that the USA did with the intrusions to the Soviet airspace several times : forced the Soviets to develop sophisticated sensor and missile systems .
And it works well.