Can I just interject for a minute and say that I disagree that Mystic Heart is not that useful or relevant in the new rotation? Below is a list of common Pokemon with effects:
Articuno - Chilling Sigh
Darkrai - Abyssal Sleep
Froakie - Bubble
Gengar - Sinister Fog
M Scizor EX - Iron Crusher
Malamar - Entangling Control
Regice - Ice Beam
Registeel - Forbidden Iron Hammer
Sceptile EX - Sleep Poison
Zorua - Moonless Madness
Magearna-EX's Mystic Heart Ability is a good Ability but it's only as good as the effects that it blocks.
Currently none of the "top tier" decks rely on blockable effects as part of their
core strategy*.
From your list, the Pokemon I'd expect to come across semi-regularly are Articuno, Froakie, M Scizor-EX, Regice, Sceptile-EX and Zorua. From these Pokemon, only Zorua and M Scizor-EX have effect-based attacks that are used as the main attack (and even then, you don't find Zorua attacking all that often).
Personally, I do not think it is worth benching a 2 prize liability in order to block the effects of peripheral Pokemon's secondary attacks.
Furthermore, PRC-STS is rife with Garbodor.
If it's good enough for Shintaro, it's good enough for me.
Shintaro played Magearna-EX in the
XY-STS metagame. Going into Worlds, I'm sure a lot of players were looking at Jirachi as a splashable tech for improving the Night March and Vespiquen matchups. As Shintaro was also playing DCE in his deck, it was a brilliant move to use Metal Energy and Magearna-EX to prevent his DCEs from being discarded.
Furthermore, Greninja (therefore Froakie) and Trevenant were popular choices and Magearna-EX added a bit more protection in these matchups.
I'm not convinced that Shintaro's reasons for playing Magearna-EX transfer to the
PRC-STS era and I'm even less convinced that his reasons apply to a M Scizor-EX deck.
* With the exception of M Scizor-EX and M Mewtwo-EX. However, in the case of M Scizor-EX, the blockable effect is the one that discards Special Energy. Magearna-EX would only be paired with decks running Metal Energy and, as the only meta deck running Metal Energy at present is M Scizor-EX itself, this effect is only relevant when it discards your Shield Energy. Even then, in most cases M Scizor-EX is paired with Garbodor. In the case of M Mewtwo-EX,
@PMJ has covered the reasons for not wanting to block Damage Change if you're playing M Scizor-EX.
Perhaps the one deck that might benefit from Magearna-EX and Metal Energy is M Rayquaza-EX but even then, that would only really be useful in the M Scizor-EX matchup to prevent DCEs from being discarded by Iron Crusher; when most of the time the opponent's M Scizor-EX is probably discarding your Sky Field anyway.