I have yet to see a single Heart of Kiran in the ~40 games I have played even though that card is completely insane.
Neither have I, which seems quite odd. I run it in a few decks, including 4-color PW.dec
I mean, I know it's a Mythic so it's not THAT strange to not see it, but I've had quite a few games where we both went down to 20-30 cards in deck and only my copy showed up.
I have only seen it a few times. I agree that PW.dec is probably the best home for it, but even then, they may be better choices.
I had it in my Bant w/ 6 PWs, and lots of creatures, but it just did not make the cut for me. The issue was I had 5 Vehicles that made the cut first, and I just felt like I didn't need a 6th. 2 Copter, 2 Harvester and Flagship all won a place before it.
IMO, the issue is that Harvester is just better. Crew 1 is the big difference. Heart belongs w/ Planeswalkers, but it is only good if you have a PW out or a 3 power creature, otherwise, it is a dead card. In most cases, I would rather have removal.
I kinda look at it like Oath of Lilliana. If you play a PW while it is already out, you have a blocker to help defend the PW. Oath kills a creature and can make a blocker for each PW, but they don't stop fliers. Heart does stop fliers, and is generally better then just a chump blocker, but does require a loyalty to activate.
By no means am I saying it is bad, it is incredible as PW defense, it just may be a bit too situational to see as much play as expected.