thats the thing i guess. its only 3 turns, Which means the Last Turn of Sun could be will be used to set Tailwind up Again, when you could get one final hit off on a switch. Flame Charge also gives ZardY a Mixed option as well as a boost, Even if it sucks. still it IS Sun Boosted. its probably pitiful damage (Haven't calced) but the blobs at least would have to think twice about switching in....kinda. Not really because of that HP.Honestly i'm still on the fence on how to boost ZardY. As you said Flame Charge sucks. Tailwind has to be set up repeatedly, which also stings. however i can see its viability, mainly because things that would switch in on ZardY need to be taken care of or active when it comes in. Politoed and Friends stop its sweep by cutting its weather, with its best check, Solarbeam being Useless along with the fact he's about to be smacked with Rain Boosted Water moves, Or statused in Hippowdon's case (Hail isn't an Issue obviously) or phased or..well as we know it doesn't like these guys. I'd say Tyranitar is its worst enemy. The Worst Heatran can do is phase. Tyranitar, Running Assualt Vest in Sand could probably Tank a focus Blast (Not well, it won't do much else after killing Zard) and smack it with a Stone Edge. I don't know the calcs but That thing is so monstrous i wouldn't doubt it.
And as much as i hate Focus Miss, your right i guess. you know if Dragon Pulse hadn't been nerfed i'd say it would have had a shot at Dealing with them, (Mainly Latios and Dnite with multiscale broken. And Chomp Obviously) but hypothetical theories will get us nowhere.
I'm think as i'm typing so this probably all sounds horrible haha. but As good as ZardY looks, ZardX looks better (Although the Flame Charge Arguement stands for DD, it needs 2, but that Attk Boost helps. alot) which is a blessing in disguise for ZardY. ZardX beats TTar not Carrying Assault Vest or Scarf (Possibly with both) and Heatran Minus Balloon (Is that still a thing? i'd assume so) meaning their less likely to switch in. so ZardY will get its chance to shine. Zard is pretty much uncounterable until you know what Mega your dealing with. Or it switches in with Rocks up
Tailwind does only last for a set number of turns, but as a Mega Charizard if you are not switched out or dead in 4 turns, you have either A). won the game or B). put your opponent at a 3-4 Pokemon deficit, which is something most opponents are unlikely to come back from and have for all intents and purposes probably won the game anyway unless your opponent is a Pokemon genius, you really goof up hard somewhere, or if you yourself were down 3+ Pokemon. On the flip side, if you have to switch out because of a counter, allow me to present to you this hypothetical scenario:
Let's say you just brought in your MegaZard in on something like a Ferrothorn. Because your opponent already knows you have a MegaZardY, he switches out for his Tyranitar. I could Focus Blast, but what if I don't feel comfortable about my chances of hitting (or what if I elect to carry Dragon Pulse instead, because some people will elect to do that)? I use Tailwind on the switch in. Turn 2, I have 4 turns of Tailwind left. I switch to my Garchomp as he Stone Edges, taking pittance damage. Turn 3, 3 turns of Tailwind left. No TTar is going to stay in on a Garchomp, so he switches. I set up Swords Dance as he brings something out. On turn 2, with 2 turns of Tailwind left, I have a +2/+2 Garchomp out on the field. I can't think of many Pokemon that can take hits from that kind of monster (Togekiss, but even then you can run Stone Edge or Rock Slide over Fire Fang/Blast). In two turns, you can take out at least 2 Pokemon and already put yourself at a pretty decent advantage even if your Tailwind will peter out in two turns and they bring in something to check your +2 Chomp.
Now this is a hypothetical. I'm not saying things will work out this well in all matches - there's so many variables in matches that the possibility of what could transpire in a match in mind blowing. I just wanted to throw out a potential opportunity to net you a significant advantage in the match using Tailwind from even ground which wouldn't have been possible with Flame Charge. Your counterpoint is still true, Flame Charge will allow Zard Y to sweep through more than 4 members of a players team, but to that I present the argument in my first paragraph.
I didn't however think of Flame Charge being used against Blissey. But I did run the damage calcs and the results aren't particularly impressive.
0 Attack neutral Nature Mega Charizard vs 0 HP/252 Def Calm Blissey (as per the standard on the Smogon page): 167.25-194.74 (A 3-4 Hit KO with Stealth Rocks up, otherwise a solid 4 hit KO no matter what)
Compared to Fire Blast's damage:
252 Special Attack +Nature Mega Charizard vs 0 HP/252 Sp Def Calm Blissey: 202.39-238.11 (a 3-Hit KO regardless of Rocks)
Now with Flame Charge you also get added utility of getting the +1 speed boost, but if you stay in 3-4 turns necessary to KO Blissey, you're just getting to get Thunder Waved. Fire Blast doesn't deal with Blissey all that much better, so a switch will be necessary unless Blissey has taken some prior damage.
I don't say this to discredit Zard Y. In fact I personally love Zard Y more than Zard X and will be using it on my team. But it can't sweep, not in the way traditional sweepers can. Zard X is much better for this as you've said. Zard Y will be one hell of a wallbreaker though, Flame Charge, Tailwind, Roost, or otherwise.
I like Flame Charge on Y simply because it's a lasting charge, and unless we find a 100+ scarfer, it's going to outspeed most things.
It's not always the best option to scarf things, either.
Off the top of my head, I can think of Scarf Terrakion, Scarf Gengar, Scarf Keldeo, Scarf Thundrus-T, and Scarf Garchomp which are all over base 100 and have been known to sport Choice Scarves every now and again. I dunno what the meta for Gen 6 will bring (such as Gengar possibly forgoing every item for Gengarite since its Mega Evolution is so powerful), but 101+ Scarfers exist and are more common than you'd think.
EDIT: In response to MrGlass, I also haven't thought about the possibility of using it to break sashes/multiscale. I know breaking multiscale with it probably won't be of much use because Dragonite is the only Multiscale user and you still can't 1HKO Dragonite after breaking Multiscale and it will just Thunder Wave or kill you outright. Breaking a Sash or Sturdy however is pretty nice. I was going to use Roost since Tailwind doesn't come out till the 25th, but you've convinced me to at least give Flame Charge a chance.
