The Dark Seal for Zyra Support

Hey, guys. So, I just got out of a game where my bot lane was snowballing at the beginning of the game. I was playing Zyra, so I figured a little extra damage wouldn't hurt anyone. In turn, I bought The Dark Seal for a little extra oomph. I had quite a bit of stacks, so I upgraded to Mejai's Soulstealer and proceeded to get 20 stacks.

Mind you, I upgraded my support item to Frost Queen's Claim and had Sightstone, control wards, and Sweeping Lens / Oracle's Alteration as every support should (aside from Frost Queen's Claim, but you know what I mean), and Mejai's was upgraded after this.

The enemy had a Leona and a Garen that were mega tanky on their team, so I figured, hey, maybe I should build a Liandry's Torment for magic pen, and I bought Rylai's Crystal Scepter for additional peel.

I am nearly mastery level 7 on Zyra, and I have had nothing but success with that build path (including this game, even though we lost; my team wasn't good at focusing the carries). Our mid laner told me to sell my things and buy utility, because I was building "mid-lane Zyra." Am I missing something? Is this not the standard build on support Zyra anymore? And who plays Zyra mid anyways? Certainly no one I've ever played with/against. I was asked to play Zyra — NOT a tank — not a healer — etc. She's really one of a kind.

But I think the thing that got to her the most was the fact that my Mejai's was stacked. Mind you, the mid-laner was Karma, so couldn't they have opted for the more utility-like build anyways because of their SHIELD? Their enemy laner Anivia kicked their ass anyways. Maybe they were tilted that I had the balls to buy The Dark Seal after my support item and Sightstone because I was that confident with my Zyra skills.

I say all this to ask... Is it "okay" to buy The Dark Seal on Zyra (or any aggressive AP supports, really)? It only costs 350 gold, and I think it pays off in the long run if you get the stacks, like I did. If not, why is it so wrong?