Do you even math bro? Here let me spell it out since you're simple. Leveling roulette can queue you into any dungeon or trial, including Satasha, for instance. You get the most exp from running the highest level dungeon you can run. Let's say that takes 30 minutes, we won't even count queue times. To compensate for level difference there is a fairly large exp bonus on running a lower level instance however individual kill exp does not change, just the end bonus. So, let's say Satasha takes 20 minutes and ifrit normal mode takes 5. The individual kill exp for a lvl 59 character in satasha adds up to a negligible, I'd go so far as to say invisible amount. The roulette bonus may fill 20% of your bar though, whereas the ifrit normal filled 18% (due to it being slightly higher level). 18% for 5 minutes is better than 20% for 25 minutes. Do you get it now?