I think a big factor into why so many people are picking it up is the novelty factor. I'm not saying that Runeglaive Ezreal isn't strong as it does help him in many of the areas he is lacking in, waveclear, Luden's not procing off of Q, mana issues etc, but I don't think his strength is the only reason we are seeing it. (As an aside I think he is REALLY strong right now) A lot of people just like the idea that they are doing something new and exciting, maybe even thinking they are breaking the meta with smite in the mid lane, even though it is kinda the meta right now to take Ezreal mid with smite and they are actually just following a trend.
It is also an exploit. People in League love exploits of design flaws. We've seen this before with the Red Smite Warwicks earlier this season/preseason where his Ult would basically one shot people when he smote them. Right now, Ezreal with Runeglaive is the big exploit. It will probably die off a little after the change to the ability to proc Luden's off of the spellblade on Runeglaive. It would still be pretty strong, but not exceedingly more powerful than AP Ezreal was before this trend. (The buff to Runeglaive may counter this, but if it remains too powerful, Ezreal's AP scalings may need to be hit, sadly)
As far as Runeglaive is concerned. I actually think it's a pretty good item, contrary to what a lot of people think. I do think that they should have kept Magus with Runeglaive as another AP option for those who do not use AAs in their rotation; RIP Fiddles. Runeglaive is just under the mark right now in my opinion and the buffs on the PBE should give it a little more oomph to junglers who might want to buy the item. (I would think giving it 60 AP would have been enough, so we'll see what 50 and a buffed spellblade will do).
Nidalee is almost guaranteed to come back into the meta, both in soloqueue and in competitive, and we might see more AP Eve's rather than the AD version we see right now, mostly in soloqueue. Diana is going to be wrecking solo queue as well and I wouldn't be surprised if she receives nerfs sometime in the future. Elise is troublesome right now as she won't be getting looked at again until 5.14 and she's in a bad spot. It's very debatable whether Runeglaive or Cinderhulk is better on her. My money is on Cinderhulk, but I have seen some Korean/European players make Runeglaive work very effectively (though they are Elise mains, and other Elise mains prefer Cinderhulk over it).
Anyway, that's my opinion on the matter. The item caters more to most of the stronger AP junglers out there (again RIP Fiddle), but it naturally might be strong on champions that thrive on Spellblade effects. We will see what happens in the future. Peace.