Ap Ezreal Smite?

Runeglaive makes his Q deal fully magic damage instead of a mix of physical and magical. There's also currently a bug that causes it to proc Luden's Echo despite being an autoattack. That's fixed on the PBE and will be in the next patch.

Apart from that, AP Ezreal's four big weaknesses have always been:

• Terrible waveclear
• Weak early-game
• Extremely squishy
• Entirely skillshot dependant

Runeglaive with Blasting Smite helps with the first two points by giving him more waveclear and mana sustain.

Its something the pros have been doing with the release of the new AP jungle enchant.

And you know the community here, they blindly follow everything the pros do. I bet half the people who go that build don't even know why its good.