Based on? How many RNG's have you seen in meripos? How many times have you merited on RNG?
How can a job that rarely ends up in a meripo be a premium meripo job? If it was a premium job for meripo it should be as frequently seen as SAM, WAR, DRG and DRK.
The fact is, while it can do comparable to superior damage to those "big 4", it has certain disadvantages that lead it to be second class.
Cost is one of the bigger reasons. Why pay for damage when you can get it for free with one of the other 4 jobs.
Poor defense is another. Even if you are meticulous about shadows or 3rd eye, you are going to get hit and you will get hurt more than a WAR/SAM/DRG leading to more MP useage.
Then there is the mob movement issue which can only be ameliorated by either standing in melee range and nerfing damage, or managing your hate and again nerfing your damage.
And finally, many buffs that BRDs and COR's like to use don't help RNG damage. Marches, Madrigals, Fighters Roll are all pointless for the RNG.
Sure none of those things will kill a meripo but they are enough to lead party leaders to think twice about getting a RNG when another of the big 4 is available and cost is enough of an issue to keep RNG's in the closet in favor of a cheaper 75 meripo job.