Are you even a midlaner? Because stop right now. You are an idiot. Point blank.
I don't build mana regen anymore because I learned to manage with just a doran's ring and rush luden's on some mages. But when I did start building Frost Queen's Claim, it was not for the passive. And many, if not all, midlaners (including lcs mid laners) build it for the fact that when you do poke and the passive is still up, that's a free 15 gold right there.
As a midlaner, you do want to poke your enemy midlaner as much as possible before going all in. Not only that, but you get cdr, a slow for an active, and mana regen (it was nice when it was 150% but 100% is nice too especially when it's much cheaper than morellos or athene's), and the 50 ap isn't that bad.
Compare it to Athene's at the price of 2800. Frost Queen's Claim costs 2200. The difference is only 10 ap, some magic resist, and 10% cdr. But that's the difference of 600 gold, that you can generate with FQC. This item is beneficial since ap items cost a hell of a lot. Many mid lane mages cannot even afford to go with deathcap unless it's late game because it's just too expensive. This helps alleviate that. Late game, you can sell the item, but not before generating like 2k in gold or more and then selling it gives back around 800 gold.
So considering you basically main champs for top lane, shut up about the lanes you DON'T KNOW. And I'm pretty sure many mid laners are smart enough to be able to read the fine print of the item they're buying and smart enough to understand what it means.
Again: FQC isn't for the passive. That's an added bonus. The active slow on it opens doors for champions like Syndra, Lux, Ahri, etc. to land their skillshots (stun, snare, charm respectively). So please, educate yourself before you try to educate others with your stupid public announcement that many others really shouldn't read.