Right now, for endgame level 50 characters, we have Rikti mothership raids and Hamidon raids which are pathetic jokes compared to live and are now essentially farmed, Incarnate trials, and advanced mode TFs/SFs that 95% of the playerbase are gatekept from. Even a few iTrial leaders do some gatekeeping by saying "No Masterminds," or "no pets," even though Masterminds are actually one of the best ATs in the game for iTrials. A new, advanced level 50 raid would be beneficial because it'd be really, really hard to gatekeep away "less desirable" ATs and power sets, and it'd actually offer something resembling a challenge unlike the current two raids.
Plus, advanced mode Rikti were designed specifically for level 50 characters.