RNG is good and creates long term goals that feel GOOD when you accomplish what you wanted.
It took me 9 months to catch Problematicus and when I did finally catch him I got the same rush as when I clear Ultimates. I felt the same way when I finally got the Heavenly Onyx Cloud Serpent in WoW. If it were up to me, I would put more RNG rewards (like rare mounts from FATES!) in the game except for stat rolls/crits because that gets very aggravating as it makes personal skill less valuable and starts to negatively affect your raids.