Then to be supersonic is necessary for a light fighter/bomber? Why?
Speed does not make any aircraft safe, but in terms of effectiveness a plane that can get to a target faster and hit it earlier and then get back to base to rearm and refuel speed can be important.
Speed wont make a small light fighter safe from being shot down by ground or air defences, but in an interceptor a faster... ie supersonic fighter can intercept targets further from their targets reducing the chance that those attack aircraft have already deployed their weapons (ie like anti ship missiles or land attack cruise missiles) and are on their way out of the area.
Also in air to air combat being able to fly at supersonic speeds even just for short periods can allow a Yak-135 to climb to high altitude and fly at supersonic speed before launching an air to air missile greatly increasing its speed and range, compared with the same missile fired from lower altitude and lower speed.
In the light attack role it means that though the aircraft might not be supersonic fully armed, it would allow a higher speed exit from the danger area.
Nothing in this life make you safe, but somethings like the speed in the air (and the armour on land) make you safer in contested areas.
It is like tank armour... no level of speed (or armour) will make you completely safe and the question comes down to cost and role. A light recon unit can't afford to have heavy armour as it would slow it down, and you couldn't afford to have every vehicle in your land fleet with MBT level armour... the fuel costs alone would be crippling...
For a fighter or a bomber having supersonic speed capability is useful SOMETIMES... there are likely some fighters and even bombers that never fly at top speed operationally because it takes time and fuel to reach top speed and few planes can maintain top speed for very long without using up all their fuel anyway.
For a trainer there are no reasons to fly supersonic... it burns fuel and limits you manouver performance and makes you an easy target for an IR guided missile.
In some comments it seems that to make a supersonic version of the Yak-130 is easy and cheap, in other comments seems that is expensive... The point of my comment was that if it is cheap, I would see interesting to have some supersonic trainers to give to the pilots their first supersonic experiences instead of doing it in more expensive aircrafts. If the Yak-135 is developed as a supersonic version of the Yak-130 it would be interesting to take advantage of it also for training purposes.
The experience of supersonic flight consists of flying up to medium altitude and then going full AB for a few minutes... odds are most people would not notice the transition from subsonic to supersonic flight except by looking at the instruments. It is actually rather a non-event... and nothing you would actually need any training in.
The concept of a trainer aircraft is to give experience to the pilots wasting less money in operational terms and risking less money in terms of equipment. It is also to avoid accidents in more expensive aircrafts. Not all the trainer aircrafts need to be supersonic.
What I am trying to suggest is that there is no real training value in being able to break the speed of sound and in a trainer aircraft the value of supersonic flight is close to zero if not actually zero.
Where supersonic speed becomes useful is in a small light aircraft with radar and self defence capability that could be used as both a trainer and a light fighter/bomber.
In such a case supersonic speed even if it is very rarely used is useful when it is needed.