In order for a SAM/RNG to hit Sidewinder reasonably well, they need a ton of RACC which they don't have as much access to. So while the RNG is stacking RATT and STR, the SAM needs to stack RACC, diminishing damage despite more freq WS. Soboro SAM/RNG is fun but it will be superceded in damage by a proper SAM/WAR with a magian weapon.

As far as RNG in endgame, the job will likely have to wait until everyone leaves Abyssea for the "Next Great Thing" TM.
Inside Abyssea its role is quite limited. I haven't been out on RNG in months.