I understand it's not actually a raid but if we're being really specific, Weeping City isn't in Raid Finder either... It would make a lot of sense for the non-DF 8-mans to be placed into RF for the patch that they are introduced in, and moved to DF when a new 8-man trial comes out.
The issue that RF was trying to solve was the fact that some servers simply did not have the PF population to be able to clear the harder content without a static. Now that only Savage is in RF, it creates the same issue, but with Nidhogg's Rage. I'm not even saying that you should be forced to do Nidhogg farms with only randoms in RF instead of partying up with people, but if you know competent people that have been past towers, you could grab those friends and queue for practice in RF at Phase 3 or so. A lot of good work went into Raid Finder and it is not seeing as much use as it could with this duty missing from it.