Who else benefits from Runeglaive?

Couldn't agree more. Except for one thing.

It used to go: Magus-->Catalyst/Tear-->Damage item now you can go: Runeglaive-->Damage Item-->Damage item

The old Magus forced you to hold off on building damage immediately as you needed mana sustain in the jungle even with Ranger's Trailblazer (bonus points on Runeglaive as you can get Red or Blue Smite now without hurting you in the jungle). You of course could horde blue buffs, but then your laners would be effectively weaker by midgame. You can still build a mana sustain item in addition to Runeglaive if you really feel the need (though it is not completely necessary), and with the AP itemization changes, AA staff and ROA are looking pretty appealing on someone like Nidalee, who could use the mana for extended sieges.

The comparison to Lichbane has been made countless times, but you both get the spellblade earlier, AND it doesn't compromise your build order if you were to rush Lichbane. For instance, with Magus, when would you be able to build Lich? Well, you'd get your jungle item first, then your mana sustain item. Next you probably want some combat stats. You could opt for Lichbane here, but an item with some defensive properties would probably be better at this point because teamfights are about to go down, so you would probably opt for Zhonya's or Abyssal as your 3rd item. Next you are going to need some meaningful damage, so your next item will probably be Deathcap or Void, but you only have 2 item slots left. Do you get Void + D-cap, Void + Lichbane, or D-cap + Lichbane?

There were big tradeoffs anyway you would build. Runeglaive solves that for the most part by taking Lichbane out of the equation, and to an extent, it takes the mana sustain item out of the equation as well, freeing up 2 slots to use as you wish. You can focus on what can give you an edge without hindering your build so much. For instance, now Nidalee can more easily buy Luden's for poking down enemies from afar, or she can emphasize her Cougar form so she can be more adaptive in teamfights. Diana can opt for a tankier build without sacrificing her build substantially, or she can build more damage than before. AP Eve looks even more viable as her E will do magic damage (on the first hit), and her AP bruiser build looks just about as good late game as her AD build with a stronger early game and more burst. Ekko and Fizz like the Sheen early as well (although I think it's very debatable if Cinderhulk+Sheen item builds are stronger on them right now). And Elise.... we won't talk about Elise :P.

I will say that Fiddle, Karthus(never saw him anyway), and AP Amumu have been hurt by the change from Magus, but all of the other prominent AP junglers have benefited from the change. I kind of feel like they should have kept Magus in conjunction for the pure Mage junglers, but I really like the new item. One thing that people fail to realize is how much more money you make with Runeglaive when compared to Magus. You can stay in the jungle longer and you clear faster. Nidalee and Diana can get as much gold as a laner just from farming in the jungle (this kind of applies to all of the AP junglers, but I feel these two have the fastest clears and can farm longer than someone like Eve who has no form of sustain or damage mitigation, so she should be looking to gank a lot early before getting some defensive/sustain oriented items).

Anyway. This is just my opinion, but I feel like I can back up my argument fairly well. I will say that the AP it gives/gave initially was very lackluster and I'm glad it is getting an AP buff (I wanted 60AP, but I will settle for 50 and a stronger spellblade). The item itself is a very good concept and I think for the most part, it delivers. Peace.