I honestly think SE knows this, and that it doesn't really belong in Trials roulette, but I think they put it in there just because they also know it is the only way they could ever get anyone to do this fight again after doing it for the story clear and they know EVERYONE needs it to enter Ishgard. *conspiracy theory*
Uhh, it's pretty far from being the only way. Ever heard of loot, perhaps minion drops?