Good idea in theory (especially if there was a prestige mount tied to clearing 100 EX Roulette in addition to tomes and more primal drops), but would require a lot of tuning and restrictions to work as intended.

Garuda/Ravana/Lakshmi EX are by and far MUCH easier than Sephirot/Thordan/Ramuh EX. It would easily turn into people fishing to do the easy ones only for the rewards. SE would have to enforce a high ilvl to queue so people don't drop their gear to only get the lower ARR primals. They would also have to either implement scaling rewards (i.e. Garuda gives 1 run towards the run 100 achievement, but Thordan would give 10) or heavy-handed penalties for dropping (i.e. 3 hour penalty instead of 30 min).

If it's just tomes and there is no said heavy-handed restrictions, it's going to turn into Garuda EX roulette.