Skill trial is rng, because it offers a cheaper and faster alternative to buying via fragments. You can buy sasuke frags in black marget for 20/1 (not that anybody sane would do it) or any ninja really. If you would want to buy books by frags, it would cost you 50 frags+, which is between 1.5-3k per book. Now single use of skill trial is 20cp. Thats cheaper than 1 frag of ninja in every case. Later on, for rare ninjas you will need other materials, but at the same time ninja frag costs goes up (propably up to 200) which is still worth it in most cases. Currently for tsunade and sasuke, if you start from 0 its better to use skill trial to hit some upgrades and buy rest with free frags, but such strategy wont really apply to meta ninjas when they will be introduced.